diff --git a/locale/ja-JP.go b/locale/ja-JP.go
new file mode 100644
index 0000000..ce18ac0
--- /dev/null
+++ b/locale/ja-JP.go
@@ -0,0 +1,38 @@
+package locale
+
+import (
+	"fmt"
+	"math"
+	"time"
+)
+
+var japaneseJapan = Formatters{
+	-1 << 63:                    func(d time.Duration) string { return fmt.Sprintf("%.0f年後", math.Ceil(-d.Hours()/(24.0*30*12))) },
+	-17 * (24 * time.Hour) * 30: func(_ time.Duration) string { return "1年後" },
+	-10 * (24 * time.Hour) * 30: func(d time.Duration) string { return fmt.Sprintf("%.0fヶ月後", math.Ceil(-d.Hours()/(24.0*30))) },
+	-45 * (24 * time.Hour):      func(d time.Duration) string { return "1ヶ月後" },
+	-25 * (24 * time.Hour):      func(d time.Duration) string { return fmt.Sprintf("%.0f日後", math.Ceil(-d.Hours()/24.0)) },
+	-35 * time.Hour:             func(_ time.Duration) string { return "1日後" },
+	-21 * time.Hour:             func(d time.Duration) string { return fmt.Sprintf("%.0f時間後", math.Ceil(-d.Hours())) },
+	-89 * time.Minute:           func(_ time.Duration) string { return "1時間後" },
+	-44 * time.Minute:           func(d time.Duration) string { return fmt.Sprintf("%.0f分後", math.Ceil(-d.Minutes())) },
+	-89 * time.Second:           func(_ time.Duration) string { return "1分後" },
+	-44 * time.Second:           func(_ time.Duration) string { return "数秒後" },
+
+	44 * time.Second:           func(_ time.Duration) string { return "数秒前" },
+	89 * time.Second:           func(_ time.Duration) string { return "1分前" },
+	44 * time.Minute:           func(d time.Duration) string { return fmt.Sprintf("%.0f分前", math.Ceil(d.Minutes())) },
+	89 * time.Minute:           func(_ time.Duration) string { return "1時間前" },
+	21 * time.Hour:             func(d time.Duration) string { return fmt.Sprintf("%.0f時間前", math.Ceil(d.Hours())) },
+	35 * time.Hour:             func(_ time.Duration) string { return "1日前" },
+	25 * (24 * time.Hour):      func(d time.Duration) string { return fmt.Sprintf("%.0f日前", math.Ceil(d.Hours()/24.0)) },
+	45 * (24 * time.Hour):      func(d time.Duration) string { return "1ヶ月前" },
+	10 * (24 * time.Hour) * 30: func(d time.Duration) string { return fmt.Sprintf("%.0fヶ月前", math.Ceil(d.Hours()/(24.0*30))) },
+	17 * (24 * time.Hour) * 30: func(_ time.Duration) string { return "1年前" },
+	1<<63 - 1:                  func(d time.Duration) string { return fmt.Sprintf("%.0f年前", math.Ceil(d.Hours()/(24.0*30*12))) },
+}
+
+func init() {
+	Register("ja", japaneseJapan)
+	Register("ja-JP", japaneseJapan)
+}
\ No newline at end of file
diff --git a/timediff_ja_JP_test.go b/timediff_ja_JP_test.go
new file mode 100644
index 0000000..84a4e9f
--- /dev/null
+++ b/timediff_ja_JP_test.go
@@ -0,0 +1,92 @@
+package timediff_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/mergestat/timediff"
+)
+
+var fixtures_ja_JP = map[string]string{
+	"-10s":                            "数秒前",
+	"-44s":                            "数秒前",
+	"-45s":                            "1分前",
+	"-89s":                            "1分前",
+	"-90s":                            "2分前",
+	"-91s":                            "2分前",
+	"-2m":                             "2分前",
+	"-10m":                            "10分前",
+	"-44m":                            "44分前",
+	"-45m":                            "1時間前",
+	"-60m":                            "1時間前",
+	"-1h":                             "1時間前",
+	"-80m":                            "1時間前",
+	"-89m":                            "1時間前",
+	"-90m":                            "2時間前",
+	"-2h":                             "2時間前",
+	"-20h":                            "20時間前",
+	"-21h":                            "21時間前",
+	"-21h30m":                         "1日前",
+	"-22h":                            "1日前",
+	"-24h":                            "1日前",
+	"-24h30m":                         "1日前",
+	"-34h59m":                         "1日前",
+	"-36h":                            "2日前",
+	fmt.Sprintf("-%dh", 10*24):        "10日前",
+	fmt.Sprintf("-%dh", 25*24):        "25日前",
+	fmt.Sprintf("-%dh", 26*24):        "1ヶ月前",
+	fmt.Sprintf("-%dh", 45*24):        "1ヶ月前",
+	fmt.Sprintf("-%dh2m", 45*24):      "2ヶ月前",
+	fmt.Sprintf("-%dh", 46*24+1):      "2ヶ月前",
+	fmt.Sprintf("-%dh", 80*24):        "3ヶ月前",
+	fmt.Sprintf("-%dh", 9*24*30):      "9ヶ月前",
+	fmt.Sprintf("-%dh", 10*24*30):     "10ヶ月前",
+	fmt.Sprintf("-%dh1m", 10*24*30):   "1年前",
+	fmt.Sprintf("-%dh", 12*24*30):     "1年前",
+	fmt.Sprintf("-%dh", 17*24*30+1):   "2年前",
+	fmt.Sprintf("-%dh", 24*24*30):     "2年前",
+	fmt.Sprintf("-%dh", 20*24*30*12):  "20年前",
+	fmt.Sprintf("-%dh", 100*24*30*12): "100年前",
+
+	"10s":                            "数秒後",
+	"44s":                            "数秒後",
+	"45s":                            "1分後",
+	"89s":                            "1分後",
+	"90s":                            "2分後",
+	"2m":                             "2分後",
+	"10m":                            "10分後",
+	"44m":                            "44分後",
+	"45m":                            "1時間後",
+	"60m":                            "1時間後",
+	"1h":                             "1時間後",
+	"80m":                            "1時間後",
+	"89m":                            "1時間後",
+	"89m10s":                         "2時間後",
+	"90m":                            "2時間後",
+	"2h":                             "2時間後",
+	"20h":                            "20時間後",
+	"21h":                            "21時間後",
+	"21h30m":                         "1日後",
+	"22h":                            "1日後",
+	"24h":                            "1日後",
+	"35h10m":                         "2日後",
+	"36h":                            "2日後",
+	fmt.Sprintf("%dh", 10*24):        "10日後",
+	fmt.Sprintf("%dh", 25*24):        "25日後",
+	fmt.Sprintf("%dh", 26*24):        "1ヶ月後",
+	fmt.Sprintf("%dh", 45*24):        "1ヶ月後",
+	fmt.Sprintf("%dh1m", 45*24):      "2ヶ月後",
+	fmt.Sprintf("%dh", 46*24):        "2ヶ月後",
+	fmt.Sprintf("%dh", 80*24):        "3ヶ月後",
+	fmt.Sprintf("%dh", 9*24*30):      "9ヶ月後",
+	fmt.Sprintf("%dh", 10*24*30):     "10ヶ月後",
+	fmt.Sprintf("%dh1m", 10*24*30):   "1年後",
+	fmt.Sprintf("%dh", 12*24*30):     "1年後",
+	fmt.Sprintf("%dh", 24*24*30):     "2年後",
+	fmt.Sprintf("%dh", 20*24*30*12):  "20年後",
+	fmt.Sprintf("%dh", 100*24*30*12): "100年後",
+}
+
+func TestTimeDiffJaJP(t *testing.T) {
+	execFixtures(t, fixtures_ja_JP, timediff.WithLocale("ja-JP"))
+}