POWER SUPPLY SYSTEM

Disclosed is a power supply system of a hydraulic unit comprising a hydraulic pump (32), a pump electrical motor (33), a hydraulic actuator (35), a main battery (41), a sub-battery (42), an inverter (43) for converting DC power from the main battery (41) into AC power and supplying the AC power to the pump electrical motor (33) and thereby operating the pump electrical motor (33), and a controller (44) which is operated by receiving DC power from the sub-battery (42), wherein there is provided a relay (47) for switching on or switching off charging of the sub-battery (42) from the main battery (41), the controller (44) detects the remaining amount of electricity in the sub-battery (42), and the relay (47) is switched on when the remaining amount of electricity falls below a predetermined value.