An engine starter and electric generator system transmits rotative power to a crankshaft to start an engine and generates electric power based on rotative power from the crankshaft after the engine has started. The engine starter and electric generator system includes a starter/generator operable selectively as a starter motor to produce the rotative power and a generator for generating the electric power, and an electric power supply device for supplying electric power to the starter motor. A power transmitting mechanism operatively interconnects the crankshaft and the starter/generator, for bidirectionally transmitting the rotative power between the crankshaft and the starter/generator. A transmission mechanism is disposed in the power transmitting mechanism, for changing the speed of rotation transmitted between the crankshaft and the starter/generator. The system also has a control device for controlling operation of the starter/generator and establishing different speed-reduction ratios for the transmission mechanism when the starter/generator operates as the generator and when the starter/generator operates as the starter motor.