In stopping a rotary body of an electric rotary excavator (a construction machine), a control-system changing unit of a rotation control device changes a control law from a speed control law to a position control law when a target speed is judged to be smaller than a speed threshold value. By changing to the position control, a larger braking torque can be output to an electric motor as compared with that in the speed control, thereby reliably maintaining the rotary body in a stationary state.