A hybrid construction machine comprising: a prime mover; a hydraulic pump which is driven by the prime mover; a swing structure; an electric motor for driving the swing structure; a hydraulic motor for driving the swing structure, the hydraulic motor being driven by the hydraulic pump; an electricity storage device which is connected to the electric motor; a swing control lever device which is operated for commanding the driving of the swing structure; and a control device which selects either a hydraulic/electric combined swing mode for driving the swing structure by total torque of the electric motor and the hydraulic motor by driving both the electric motor and the hydraulic motor when the swing control lever device is operated, or a hydraulic solo swing mode for driving the swing structure by the torque of the hydraulic motor alone by driving only the hydraulic motor when the swing control lever device is operated, characterized in that the hybrid construction machine further comprises a swing-mode switching command means which is manually operated for commanding switching between the hydraulic/electric combined swing mode and the hydraulic solo swing mode, and in that the control device includes a hydraulic/electric combined swing control unit for executing hydraulic/electric combined swing mode control when a switching signal from the swing-mode switching command means commands switching to the hydraulic/electric combined switching mode, and a hydraulic solo swing control unit for executing hydraulic solo swing mode control when a switching signal from the swing-mode switching command means commands switching to the hydraulic solo swing mode. a prime mover; a hydraulic pump which is driven by the prime mover; a swing structure; an electric motor for driving the swing structure; a hydraulic motor for driving the swing structure, the hydraulic motor being driven by the hydraulic pump; an electricity storage device which is connected to the electric motor; a swing control lever device which is operated for commanding the driving of the swing structure; and a control device which selects either a hydraulic/electric combined swing mode for driving the swing structure by total torque of the electric motor and the hydraulic motor by driving both the electric motor and the hydraulic motor when the swing control lever device is operated, or a hydraulic solo swing mode for driving the swing structure by the torque of the hydraulic motor alone by driving only the hydraulic motor when the swing control lever device is operated, characterized in that the hybrid construction machine further comprises a swing-mode switching command means which is manually operated for commanding switching between the hydraulic/electric combined swing mode and the hydraulic solo swing mode, and in that the control device includes a hydraulic/electric combined swing control unit for executing hydraulic/electric combined swing mode control when a switching signal from the swing-mode switching command means commands switching to the hydraulic/electric combined switching mode, and a hydraulic solo swing control unit for executing hydraulic solo swing mode control when a switching signal from the swing-mode switching command means commands switching to the hydraulic solo swing mode.