Provided is a construction machine capable of achieving an energy conservation effect while restraining overcharge of an electrical storage device, including an engine, a slewing electric motor, an electrical storage device, a generator-motor, and an assist controller which causes the generator-motor to assist the engine to consume an electric power equal to or more than an electric power equal to a difference between the regeneration power and the maximum allowable charge amount, when a regeneration power regenerated by the slewing electric motor is equal to or more than a maximum allowable charge amount which is a maximum amount of an electric power within which the electrical storage device can be charged.