A transmission gear for an automobile includes a dynamo-electric machine operating as a motor and a generator, a dynamo-electric machine control device, a dynamo-electric machine pulley, an auxiliary machine pulley, a crank pulley rotatable relative to or integrally with the crank shaft, a sun gear coupled to the crank pulley, planetary gears each rotatable and revolvable along an outer periphery of the sun gear, a carrier for supporting the planetary gear rotatably and revolvably and connected to the crank shaft, an internal gear meshing with the planetary gear, an electromagnetic clutch for changing over the internal gear between restrained and unrestrained states, an electromagnetic clutch changeover control device, and a one-way clutch provided between the internal gear and the carrier.