To provide a hydraulic drive type power generation device that eliminates the need to prepare a hydraulic pump exclusively for power generator driving, and obtains a predetermined numbers of revolving of a power generator without depending upon the numbers of revolving of an engine.

In a hydraulic drive type power generation device which drives a power generator by a hydraulic motor 19 revolving with pressure oil that a hydraulic pump 14 connected to an engine 12 discharges, a belt for power transmission is extended between a pulley fitted to an output shaft of the hydraulic motor and a pulley fitted to an input shaft of the power generator, and a heavy pulley with high inertia which maintains the predetermined revolving speed is used as each pulley, so that load variation exerts no influence on a hydraulic circuit.