It is intended to provide a power generating apparatus of renewable energy type which obtains desired torque rapidly in response to changes of the renewable energy as well as a method of operation such an apparatus. The power generating device 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 fluid communication with an intake side of the hydraulic motor 14, a pump demand determination unit 44 which determines a displacement demand Dp of the hydraulic pump based on a target torque Td of the hydraulic pump 12 and an oil pressure in the high pressure oil line 16 and a pump controller 46 which adjusts displacement of the hydraulic pump 12 to the determined displacement demand Dp.