The present invention provides a construction machine that is capable of incorporating an engine and using an external power source. The construction machine includes an electrical storage device (14) for performing a charge with the external power source, uses the electrical storage device to provide assist when the power of the engine (1) is insufficient, uses an electrical charge in the electrical storage device systematically in accordance with work time, and efficiently uses the electrical storage device by controlling its discharge amount in accordance with load status. In particular, the construction machine includes the engine (1), accessory loads (16) connected to the engine, an assist electric motor (2) driven by the engine, electrical power converters (9, 10, 13) for converting an AC current, which is the output current of the assist electric motor, to a DC current, and the electrical storage device (14) connected to the DC side of each electrical power converter. The electrical storage device becomes charged through a charging device (15) from the external power source.