A twin clutch type transmission connected to an engine of a vehicle and the like in which an end portion of the main shaft is held in the secure and compact manner to contribute to efficient operation of the transmission. In the twin clutch type transmission, the first and second hydraulic disk clutches are coaxially provided to be adjacent with each other, and the inner and outer shafts of the main shaft as a coaxial dual structure are connected to the respective disk clutches coaxially. The ball bearing fixed to the transmission case with the holder plate pierces the left end portion of the inner shaft so as to be supported, and further to be tightened and fixed to the ball bearing with the locknut screwed with the leading end of the left end portion.