A hybrid construction machine comprising: a vehicle body (2, 4) that is provided with a revolving structure; a working mechanism that is provided on said revolving structure; an engine that is provided on said vehicle body (2, 4); a motor-generator that is connected mechanically to said engine; an electricity storage device that is connected electrically to said motor-generator; a hydraulic pump that is connected mechanically to said engine; a plurality of actuators (12D, 12E, 12F, 25, 26) that drive said vehicle body (2, 4) or said working mechanism and include a boom cylinder and a revolving hydraulic motor that is driven by pressurized oil from said hydraulic pump; a revolving electric motor that is provided with said vehicle body (2, 4) and connected electrically to said motor-generator and said electricity storage device to revolve said revolving structure by compound torque with said revolving hydraulic motor; an actuator operation device (9, 10, 11) that drives said plurality of actuators (12D, 12E, 12F, 25, 26) in accordance with an operating amount; and a controller that controls output of said motor-generator and has a low speed mode for reducing movement speeds of said plurality of actuators (12D, 12E, 12F, 25, 26) in response to conditions of said motor-generator and said electricity storage device and a normal mode in which a reduction in the movement speeds of said plurality of actuators (12D, 12E, 12F, 25, 26) is released, characterized in that: said controller has a function of reducing the output of said boom cylinder and said revolving hydraulic motor in such a manner as to hold a ratio of the output of said revolving movement and the output of said boom-raising movement to the same value of a ratio in said normal mode at the time of performing a compound movement of a revolving movement and a boom-raising movement of said working mechanism in said low speed mode, and a function of controlling output of said revolving electric motor, and when said compound movement is performed in said low speed mode and said revolving electric motor and said motor-generator simultaneously perform powering operations, said controller increases a reduced value of the output of said motor-generator to be larger than a reduced value of the output of said revolving electric motor. a vehicle body (2, 4) that is provided with a revolving structure; a working mechanism that is provided on said revolving structure; an engine that is provided on said vehicle body (2, 4); a motor-generator that is connected mechanically to said engine; an electricity storage device that is connected electrically to said motor-generator; a hydraulic pump that is connected mechanically to said engine; a plurality of actuators (12D, 12E, 12F, 25, 26) that drive said vehicle body (2, 4) or said working mechanism and include a boom cylinder and a revolving hydraulic motor that is driven by pressurized oil from said hydraulic pump; a revolving electric motor that is provided with said vehicle body (2, 4) and connected electrically to said motor-generator and said electricity storage device to revolve said revolving structure by compound torque with said revolving hydraulic motor; an actuator operation device (9, 10, 11) that drives said plurality of actuators (12D, 12E, 12F, 25, 26) in accordance with an operating amount; and a controller that controls output of said motor-generator and has a low speed mode for reducing movement speeds of said plurality of actuators (12D, 12E, 12F, 25, 26) in response to conditions of said motor-generator and said electricity storage device and a normal mode in which a reduction in the movement speeds of said plurality of actuators (12D, 12E, 12F, 25, 26) is released, characterized in that: said controller has a function of reducing the output of said boom cylinder and said revolving hydraulic motor in such a manner as to hold a ratio of the output of said revolving movement and the output of said boom-raising movement to the same value of a ratio in said normal mode at the time of performing a compound movement of a revolving movement and a boom-raising movement of said working mechanism in said low speed mode, and a function of controlling output of said revolving electric motor, and when said compound movement is performed in said low speed mode and said revolving electric motor and said motor-generator simultaneously perform powering operations, said controller increases a reduced value of the output of said motor-generator to be larger than a reduced value of the output of said revolving electric motor.