An step-up gear unit is adapted for realization of an automotive internal combustion engine simulated driving power supply in use with motor such as an electric motor, a hydraulic motor and so forth. The step-up gear unit is connected to at least two motors for selectively outputting driving torque of sole motor and combined torque of both motors for wider capacity range of capacity of engine to simulate. The gear train of the step-up gear is so composed to selectively connect and disconnect the motors to an output gear for selective output of the driving torque.