In a dual clutch transmission, on one end side of a first input shaft connectable to an internal combustion engine via a first clutch, a planetary gear mechanism for a predetermined speed stage is provided, and a sun gear thereof is provided to rotate integrally with the first input shaft and an electric motor. A ring gear is fixed to a case, and a synchromesh mechanism is provided connectably and disconnectably with respect to the planetary gear carrier for selecting the predetermined speed stage. When the synchromesh mechanism is connected to the planetary gear carrier, rotation of the carrier is transmitted to the countershaft. The ring gear is attached to an annular attachment member to be fixed to the case. In a circumferential side surface of the attachment member, openings for inserting a shift fork and an opening for inserting a finger for the convenience of assembly work are provided.