Provided a transmission including a first shaft, a second shaft connected thereto via a shift gear mechanism provided to a first shaft and generating a rotary output corresponding to a selected shift gear, a planetary gear mechanism, a transmission case for housing the planetary gear mechanism and a mounting member for fixing a ring gear of the planetary gear mechanism to the case. To the mounting member, an abutment support portion is formed for supporting the case by abutting to an inter-shaft portion positioned between the first shaft and the second shaft in the case.