To provide a synchronous device of a transmission capable of effectively lowering peak value of operating load during synchronization.

The synchronous device provides a support synchronous force generated mechanism which converts force in peripheral direction subjected to relative rotation into support synchronous force in axial direction urging a balk ring 4 into a clutch gear 3 when the relative rotation generates between a synchronous hub 5 and the balk ring 4 in the event of transmission. The device interposes a return spring 9 returning the balk rings 4, 4' in initial position between the balk rings 4, 4' so that the support synchronous force does not generate when a coupling sleeve 1 is in neutral position and when a spline teeth in either of the coupling sleeve 1 or the clutch gears 3, 3' are engaged with each other. The device includes on both ends of the spring a pair of mounting plates 10, 10' constructed so as to interpose the balk rings 4, 4' from outside and forms on the mounting plates 10, 10' an elongated hole 10b for mounting the return spring which opens in radial direction longitudinally.