The fluid power generator is provided. The fluid power generator includes: a rotating member that rotates by fluid force; a generator motor that is mechanically linked with the rotating member and that is configured to function both as a generator and as a motor; a rotation speed meter that measures a rotation speed of the generator motor; and a controller that controls the generator motor, wherein the controller has a control mode to keep the rotation of the rotating member irrespective of a variation in flow rate of the fluid.