NOVELTY - The apparatus has a rotating shaft (8) driven by a renewable energy source. A variable displacement type hydraulic pump (12) is driven by the shaft. A variable displacement type hydraulic motor (14) is driven by pressurized oil supplied from the pump. A discharge side of the pump is in fluid communication with an intake side of the motor via a high pressure oil line (16). A pump controller (46) adjusts displacement of the pump to a pump displacement demand (Dp) that is determined by a pump demand determination unit (44) based on target torque of the pump and oil pressure in the line.    USE - Renewable energy type power generating apparatus i.e. wind turbine generator (claimed), for generating power from a renewable energy source e.g. wind power and tidal current. Can also be used for a tidal current generator installed in sea, river and lake.    ADVANTAGE - The pump controller adjusts the displacement of the pump to the pump displacement demand that is determined by the pump demand determination unit based on the target torque of the pump and the oil pressure in the high pressure oil line, thus obtaining the desired torque that is optimal torque at which the energy is extracted efficiently from the renewable energy source, while adjusting the torque rapidly to follow the fluctuations of the renewable energy by changing the target torque of the hydraulic pump in response to the fluctuations of the renewable energy. The apparatus allows determination of the ideal torque based on the wind speed measured by an anemometer, instead of determining the ideal torque by using a rotation speed meter, thus enabling the apparatus to improve power generation efficiency, while obtaining the wind speed with high accuracy by directly measuring the wind speed by the anemometer to control the hydraulic pump appropriately. The apparatus allows determination of the ideal torque of the rotating shaft based on the average of the wind speed measured by the anemometer, thus improving the accuracy of determining the ideal torque, and removing the noise due to the rotation speed meters or external factors.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for operating a renewable energy type power generating apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of a structure of a hydraulic transmission and a generator of a wind turbine generator.    Pump displacement demand (Dp)    Rotating shaft (8)    Variable displacement type hydraulic pump (12)    Variable displacement type hydraulic motor (14)    High pressure oil line (16)    Pump demand determination unit (44)    Pump controller (46)