A transmission case is provided with a housing enclosing a clutch and/or torque converter connected to an engine, and a shifting mechanism case enclosing a shifting mechanism in an oil-tight state that is connected to the housing. A focus for stress resulting from an impact load acting on the transmission case is formed as a frangible portion in the housing. Thus, damage to the transmission from an excessive load is limited to the housing, and by absorbing the impact from that damage, damage to the shifting mechanism case is prevented and oil leakage from the transmission case is avoided.