A transmission case includes a housing that accommodates a starting device, the housing having rib portions on an inner surface; and a speed change mechanism case connected to the housing, the speed change mechanism case accommodating a speed change mechanism, wherein a thin wall portion is provided in the housing by a groove formed in the housing, and the transmission case is structured such that stress is concentrated on the thin wall portion by at least one of the rib portions when an impact load is applied to the transmission case.