To form compact structure so as to be enclosable in an existing engine room by suppressing a dimensional increase in the axial direction of an engine in integrating an assisting motor generator into a power source for a construction machine comprising an internal combustion engine and a hydraulic pump.

A rotor magnet 23 is installed on the outer peripheral surface of a cylinder block 15 of a swash plate type hydraulic pump P driven by the internal combustion engine E used as the power source of the construction machine, and a stator coil 24 is installed on the inner peripheral surface of a swash plate type hydraulic pump casing 8, opposedly to the rotor magnet 23 to form compact structure with the assisting motor generator MG integrated into the swash plate type hydraulic pump P.