Abstract: Rotational output of an electric motor is directly connected to a hydraulic pump or a hydraulic motor. The electric motor acts as drive source for a hydraulic drive machine such as hydraulic shovel, bulldozer or that for a generator. A hydraulic drive device utilizing the electric motor includes an electric motor 102, a first rotation transmission device 106 connected to an output shaft 102a of the electric motor 102 and performing speed increasing function, a hydraulic pump 108 driven by an output shaft 106b of the first rotation transmission device 106, a hydraulic motor 110 where oil from the hydraulic pump 108 is supplied through an operation-switching valve 109, a second rotation transmission device 107 connected to an output shaft 110a of the hydraulic motor 110 and performing speed increasing function, and a drive output shaft 125 disposed on the output side of the second rotation transmission device 107.