The power generating device (1000) has a vane (1200) rotated by hydrodynamic force from a fluid; a generator motor (100); a flow velocity measuring unit (1130); a rotation speed measuring unit; and a control unit (200). In the event that at least one of the flow velocity and the rotation speed lies within a prescribed low range at startup of the generator motor, the control unit (200) operates the generator motor as a motor to increase the rotation speed of the vane, and then operates the generator motor as a generator.