A hybrid shovel includes an engine 11 that is controlled at a constant revolution speed, a motor generator 12 that assists the engine 11, a controller 30 that controls a drive of the motor generator 12, and a main pump that is driven by the engine 11. The controller 30 assists the engine 11 by the motor generator 12 when a revolution speed of the engine 11 is lower than said constant revolution speed due to a load of the main pump 14, and decreases an assist output to the engine 11 by the motor generator 12 before the revolution speed of the engine 11 returns to the constant revolution speed.
