PURPOSE: To smoothly start a windmill even in case of weak wind power by constituting a power transmission apparatus for the windmill from a power transmission part consisting of a tachometer and a hydraulic circuit for a hydraulic pump and a hydraulic motor, and a controller for controlling the power transmission part by the revolution speed signal.

CONSTITUTION: When a windmill 2 starts revolution by wind power, a hydraulic pump 1 is driven, but in case of weak wind, the revolution signal of a tachometer 3 is detected by a controller 11, and a flow-rate adjuster 6 is controlled, and oil is sent to a bypass circuit 4, in order to reduce the revolution load of the windmill 2. Since the oil pressure flowing in a hydraulic circuit 5 is low, also the driving torque of the hydraulic pump 1 is small, and the windmill 2 can be revolved smoothly.