A hybrid construction machine includes an engine, a hydraulic pump configured to be driven by the engine, a hydraulic actuator configured to be driven with a hydraulic pressure from the hydraulic pump, an electric motor configured to assist the driving of the engine, and a generator configured to be driven by the engine to generate power. The number of revolutions of the engine is caused to vary in accordance with the size of a load on the engine. In response to a decrease in the load requested of the engine, the engine is decelerated by causing the generator to generate power. In response to an increase in the load requested of the engine, the engine is accelerated by assisting the engine by driving the electric motor.