A synchronizing device for transmission having a clutch hub disposed to a shaft and a clutch hub groove formed thereto, a sleeve splined and connected to an external side of the clutch hub for being capable of moving along axial direction of the shaft, a blocking key disposed to the clutch hub groove, and a clutch gear disposed coaxial to the shaft, may include a blocker ring disposed to slidably touch a cone portion formed to the clutch gear, wherein a blocking key receiving portion for supporting the blocking key may be formed to the blocker ring.