A power supply system for a hydraulic unit comprising: a first battery for outputting DC power; a second battery for outputting DC power, the second battery being capable of being charged upon receiving a supply of power from the first battery; an inverter for converting DC power from the first battery into AC power, and outputting the AC power; a pump electrical motor and a revolving motor as AC electric motors (33,45), which are driven by AC power outputted by the inverter; and an inverter controller, which is operated upon receiving DC power from the second battery and adapted to control the operation of the inverter; the power supply system comprising: a charge ON/OFF switch for switching on or switching off the charging of the second battery from the first battery; and a switch controller for detecting a remaining amount of electricity in the second battery, and switching on the charge ON/OFF switch when the remaining amount of electricity falls below a predetermined value, characterized in that the power supply system further comprising; regenerated power storing means (61,62,63), which are electrically connected to the second battery and the AC electric motors (33,45), and are used to store regeneratively produced power by the AC electric motors (33,45); a regeneration ON/OFF switch for switching on or switching off the electrical connection between the regenerated power storing means (61,62,63) and the second battery; a regeneration ON/OFF switch operation unit for switching the regeneration ON/OFF switch on or off based on a judgement whether the AC electric motors (33,45) are in a driving state or a stopped state; and when the regeneration ON/OFF switch operation unit switches on the regeneration ON/OFF switch, the second battery is charged by the power stored in the regenerated power storing means (61,62,63), the regeneration ON/OFF switch operation unit switches off the regeneration ON/OFF switch upon judging that the AC electric motors (33,45) are in a driving state, and switches on the regeneration ON/OFF switch upon judging that the AC electric motors (33,45) are in a stopped state, and the regenerated power storing means (61,62,63) comprise capacitors (61,62,63) for storing the regenerated driving energy of the AC electrical motors (33,45). a first battery for outputting DC power; a second battery for outputting DC power, the second battery being capable of being charged upon receiving a supply of power from the first battery; an inverter for converting DC power from the first battery into AC power, and outputting the AC power; a pump electrical motor and a revolving motor as AC electric motors (33,45), which are driven by AC power outputted by the inverter; and an inverter controller, which is operated upon receiving DC power from the second battery and adapted to control the operation of the inverter; a charge ON/OFF switch for switching on or switching off the charging of the second battery from the first battery; and a switch controller for detecting a remaining amount of electricity in the second battery, and switching on the charge ON/OFF switch when the remaining amount of electricity falls below a predetermined value, characterized in that the power supply system further comprising; regenerated power storing means (61,62,63), which are electrically connected to the second battery and the AC electric motors (33,45), and are used to store regeneratively produced power by the AC electric motors (33,45); a regeneration ON/OFF switch for switching on or switching off the electrical connection between the regenerated power storing means (61,62,63) and the second battery; a regeneration ON/OFF switch operation unit for switching the regeneration ON/OFF switch on or off based on a judgement whether the AC electric motors (33,45) are in a driving state or a stopped state; and when the regeneration ON/OFF switch operation unit switches on the regeneration ON/OFF switch, the second battery is charged by the power stored in the regenerated power storing means (61,62,63), the regeneration ON/OFF switch operation unit switches off the regeneration ON/OFF switch upon judging that the AC electric motors (33,45) are in a driving state, and switches on the regeneration ON/OFF switch upon judging that the AC electric motors (33,45) are in a stopped state, and the regenerated power storing means (61,62,63) comprise capacitors (61,62,63) for storing the regenerated driving energy of the AC electrical motors (33,45).