To provide a rotation synchronizing mechanism for an automatic clutch type transmission capable of improving the friction characteristic of a rotation synchronizing brake device for synchronizing gears of a transmission when shifting up.

This automatic clutch type transmission is provided with a gear shift clutch 3 for connecting/disconnecting the power of an engine E, a transmission T/M connected to the gear shift clutch 3 for transmission, a shift lever 21 for changing a gear M of the transmission T/M, and a control device 22 for automatically connecting/disconnecting the gear shift clutch 3 with the operation of the shift lever 21. An input shaft 8 of the transmission T/M is provided with a rotation synchronizing brake device 70 for synchronizing the gear M of the transmission by reducing the rotation of the input shaft 8 when shifting up.