In view of the problems above, it is an object of the present invention is to provide a power generating apparatus of renewable energy type which can achieve a desired output of the hydraulic motor and a stable power generation regardless of changes of the renewable energy as well as a method of operation such an apparatus. The power generating apparatus which generates power from a renewable energy source, includes a rotating shaft 8 driven by the renewable energy, a hydraulic pump 12 of variable displacement type driven by the rotating shaft 8, a hydraulic motor 14 driven by pressurized oil supplied from the hydraulic pump 12, a generator 20 coupled to the hydraulic motor 14, a high pressure oil line 16 through which a discharge side of the hydraulic pump 12 is in communication with an intake side of the hydraulic motor 14, a low pressure oil line 18 through which an intake side of the hydraulic pump 12 is in communication with a discharge side of the hydraulic motor, a motor target output determination unit 45 which determines the target output of the hydraulic motor 14, POWERmotor based on the target output of the hydraulic pump, POWERpump, a motor demand output determination unit 46 which determines a displacement demand Dm of the hydraulic motor 14 based on the target output of the hydraulic motor 14, POWERmotor so that the rotation speed of the generator 20 is constant, and a motor controller 48 which adjust the displacement of the hydraulic motor to the displacement demand Dm.