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 based on the target output of the hydraulic pump, 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, 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.