diff --git a/cmd/server/main.go b/cmd/server/main.go
index 72a9e39..ecb6ce3 100644
--- a/cmd/server/main.go
+++ b/cmd/server/main.go
@@ -34,6 +34,7 @@ func main() {
 	// --- DI 配線(ここだけが全層を組み立てる) ---
 	s := server.New(cfg, server.Handlers{
 		Health: handler.NewHealthHandler(database),
+		Janken: handler.NewGameHandler(database),
 	})
 	log.Fatal(s.Serve(cfg.Port))
 }
diff --git a/internal/handler/janken.go b/internal/handler/janken.go
new file mode 100644
index 0000000..6326184
--- /dev/null
+++ b/internal/handler/janken.go
@@ -0,0 +1,83 @@
+package handler
+
+import (
+	"database/sql"
+	"fmt"
+	"math/rand"
+	"net/http"
+
+	"github.com/labstack/echo/v5"
+)
+
+var winningMap = map[string]string{
+	"rock":     "scissors",
+	"scissors": "paper",
+	"paper":    "rock",
+}
+
+type GameHandler struct {
+	db *sql.DB
+}
+
+func NewGameHandler(db *sql.DB) *GameHandler {
+	return &GameHandler{db: db}
+}
+
+// リクエストボディの構造体
+type PlayGameRequest struct {
+	UserHand string `json:"user_hand"` // "rock", "scissors", "paper"
+}
+
+// 200 OK: 成功時のレスポンス
+type PlayGameResponse struct {
+	Result  string `json:"result"`   // "win", "lose", "draw"
+	CPUHand string `json:"cpu_hand"` // "rock", "scissors", "paper"
+}
+
+// PlayGame godoc
+// @Summary     じゃんけんを実行する
+// @Description ユーザーの手を受け取って、CPUとのじゃんけん対戦の結果を返してくれるAPI
+// @Accept      json
+// @Produce     json
+// @Param       request body PlayGameRequest true "ユーザーの手"
+// @Success     200 {object} PlayGameResponse
+// @Failure     400 {object} map[string]string
+// @Router      /janken [post]
+func (g *GameHandler) PlayGame(c *echo.Context) error {
+	// ユーザーの手を読み込む
+	var req PlayGameRequest
+	if err := c.Bind(&req); err != nil {
+		fmt.Println(err)
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "リクエストが不正です", "cause": err.Error()})
+	}
+
+	// rock, scissors, paper以外を弾く
+	if _, exists := winningMap[req.UserHand]; !exists {
+		return c.JSON(http.StatusBadRequest, map[string]string{"error": "rock, scissors, paperのいずれかを送信してください"})
+	}
+
+	// CPUの手をランダムに決める
+	hands := []string{"rock", "scissors", "paper"}
+	cpuHand := hands[rand.Intn(3)]
+
+	// 勝敗を判定する
+	result := getMatchResult(req.UserHand, cpuHand)
+
+	// JSONで結果を返す
+	res := PlayGameResponse{
+		Result:  result,
+		CPUHand: cpuHand,
+	}
+	return c.JSON(http.StatusOK, res)
+}
+
+func getMatchResult(userHand, cpuHand string) string {
+	ans := "lose"
+
+	if userHand == cpuHand {
+		ans = "draw"
+	} else if winningMap[userHand] == cpuHand {
+		ans = "win"
+	}
+	return ans
+}
diff --git a/internal/server/server.go b/internal/server/server.go
index 2f5f8cd..515eca1 100644
--- a/internal/server/server.go
+++ b/internal/server/server.go
@@ -12,6 +12,7 @@ import (
 
 type Handlers struct {
 	Health *handler.HealthHandler
+	Janken *handler.GameHandler
 }
 
 type Server struct {
@@ -32,6 +33,7 @@ func New(cfg config.Config, h Handlers) *Server {
 func registerRoutes(e *echo.Echo, h Handlers) {
 	e.GET("/health", h.Health.Health)
 	e.GET("/health/db", h.Health.DBHealth)
+	e.POST("/janken", h.Janken.PlayGame)
 
 	// OpenAPI 3.x (swag v2) 用の Swagger UI
 	e.GET("/swagger/*", echoSwagger.WrapHandlerV3)