To provide a main shaft drive power generator coupled with the main shaft capable of continuously switching a shaft generator between a shaft-generation mode and an electric propulsion mode.

A shaft generator 5 includes: a second rotation control circuit 15-2 that controls the number of revolutions of the shaft generator 5; and a first rotation control circuit 15-1 that controls the number of revolutions of an asynchronous phase modifier 12, which are connected to each other via an integration setter 15-10. When switching a control unit 15 from a shaft-generation mode to an electric propulsion mode for low speed navigation, the integration setter 15-10 inputs the output from the first rotation control circuit 15-1 into an integral element of the second rotation control circuit 15-2 until immediately before the switching, and immediately after the switching, controls the number of revolutions of the shaft generator 5 to start from an initial value. When switching from the electric propulsion mode to the shaft-generation mode, the integration setter 15-10 inputs the output from the second rotation control circuit 15-2 to the integral element of the first rotation control circuit 15-1 as an initial value immediately before the switching; and immediately after the switching, controls the number of revolutions of the synchronous phase modifier 12 to start from the initial value to avoid to change sharply the output current command value.