Belt drive system

A belt drive system can appropriately transmit power of an electric rotating machine without setting initial belt tension to be large. In the belt drive system, a stationary tension pulley (18) is located on the side opposite to a crank pulley with respect to an electric rotating machine pulley (13) so that contact angle of a belt that is wrapped around the electric rotating machine pulley (13) in contact therewith may be in a range of 130 DEG to 230 DEG ; an idle pulley (17a) of a first stopper-equipped auto-tensioner (17) is located between the electric rotating machine pulley (13) and an internal combustion engine crank pulley (2); and position of the idle pulley (17a) is fixed at the time of starting an internal combustion engine; while the idle pulley (17a) coming to be movable at the time of driving the electric rotating machine as a generator.