To provide a self-operated power generation device that generates power and charges a battery by revolving a water turbine with flow force of water from a water pipe or the like to rotate a generator.

An electric circuit 45 includes the generator 3 and the water turbine 2 for driving the generator 3. Pressure detecting sensors 5 and 8 are arranged respectively on an inlet side 4 and an outlet side 7 of a flow passage 1 to the water turbine 2. Out of an inflow pressure and an outflow pressure of water detected by the sensors 5 and 8, the inflow pressure is introduced into a high pressure side of a rotational frequency control valve 10 and the outflow pressure is introduced into a low pressure side of the same rotational frequency control valve 10, and a differential pressure between the inflow pressure and the outflow pressure is kept constant to hold a flow velocity constant and hold the rotational frequency of the generator 3 constant.