NOVELTY - The unit (6) has a variable displacement type hydraulic motor (20) being driven by pressure oil from a hydraulic pump (10). The hydraulic motor is configured such that displacement of the hydraulic motor is variable within a range from minimum value of the displacement of the motor to maximum value of the displacement of the motor, where the minimum value that is a lower limit of the variable range for the displacement of the hydraulic pump is greater than zero, and the minimum value which is a lower limit of the variable range for the displacement of the hydraulic motor is equal to zero.    USE - Hydraulic transmission unit for use in a renewable energy type power generating apparatus i.e. wind turbine generator (all claimed). Can also be used for tidal current power generator, ocean current power generator, river current power generator, vessel propeller control machine, civil engineering machine and construction machine.    ADVANTAGE - The unit allows displacement of the hydraulic pump to be variable in addition to the displacement of the hydraulic motor in a high load region of the unit such that freedom of controlling the unit is secured. The unit reduces a number of active control valves for the hydraulic pump compared to being provided in each of a cylinder of the hydraulic pump, thus reducing unit cost, reducing minimum discharge amount from the hydraulic pump and reducing capacity of unit for restricting an amount of oil flowing into the hydraulic motor from the hydraulic pump.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a renewable energy type power generating apparatus    (2) a method for operating a hydraulic transmission unit    (3) a method for operating a renewable energy type power generating apparatus.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of wind turbine generator provided with hydraulic transmission.    Hydraulic transmission unit (6)    Hydraulic pump (10)    Variable displacement type hydraulic motor (20)    Pump control part (52)    Motor control part (53)    Bypass control part (54)