A drive unit (1) for driving a hydraulic pump has an electric motor (2) for driving the hydraulic pump (100) of a construction working machine and also has a generator (3) for supplying electricity to the electric motor (2). A generation module (14) of the generator (3) has a structure where a large number of electrode assemblies (42) are serially connected between fastening plates (41) in a condition that partition plates (44) are sandwiched between the respective electrode assemblies (42). Liquid fuel for generating hydrogen and air are supplied to the electrode assemblies (42) to generate electric power. Unlike the case where the hydraulic pump (100) is driven by a diesel engine etc., the hydraulic pump drive unit has a low noise level and emits no exhaust gas, so that the device is extremely advantageous to reduce noise and exhaust gas of a construction working machine.