The present invention provides a transmission mounting structure. A transmission case is constructed by connecting a clutch case, a case main body, and an end case, which are split in the direction of a rotational center axis. The clutch case and the case main body are provided with projecting portions. The projecting portions are connected by co-fastening bolts. Fixtures are secured to the projecting portions by mounting bolts, which are installed from the direction of the rotational center axis. The transmission is installed to a vehicle through the intermediary of supports. The projecting portions are co-fastened by the mounting bolts.