A slewing control device for a hybrid construction machine, the slewing control device comprising: a slewing motor configured to slew a slewing superstructure; a slewing operation amount detection unit configured to detect a slewing operation amount of the slewing superstructure; a slewing control unit configured to output a slewing command for operating the slewing superstructure at a slewing speed corresponding to the slewing operation amount, thereby controlling the slewing motor; a slewing speed detection unit configured to detect a slewing speed of the slewing superstructure; a mechanical brake configured to mechanically stop and hold the slewing superstructure; characterized in that a brake control unit is configured to, when the slewing operation amount indicates slewing stop, avoid operating the mechanical brake until the detected slewing speed is equal to or lower than a predetermined speed, and operate the mechanical brake after the detected slewing speed is equal to or lower than the predetermined speed; a brake operation detection unit is configured to detect a brake operation detection value representing a braking force of the mechanical brake; and a time measurement unit is configured to measure a time period during which the detected brake operation detection value exceeds a predetermined threshold, wherein, when the mechanical brake is operated, the slewing control unit outputs the slewing command until the time period measured by the time measurement unit exceeds a predetermined reference time period, and stops outputting the slewing command after the measured time period exceeds the predetermined reference time period. a slewing motor configured to slew a slewing superstructure; a slewing operation amount detection unit configured to detect a slewing operation amount of the slewing superstructure; a slewing control unit configured to output a slewing command for operating the slewing superstructure at a slewing speed corresponding to the slewing operation amount, thereby controlling the slewing motor; a slewing speed detection unit configured to detect a slewing speed of the slewing superstructure; a mechanical brake configured to mechanically stop and hold the slewing superstructure; characterized in that a brake control unit is configured to, when the slewing operation amount indicates slewing stop, avoid operating the mechanical brake until the detected slewing speed is equal to or lower than a predetermined speed, and operate the mechanical brake after the detected slewing speed is equal to or lower than the predetermined speed; a brake operation detection unit is configured to detect a brake operation detection value representing a braking force of the mechanical brake; and a time measurement unit is configured to measure a time period during which the detected brake operation detection value exceeds a predetermined threshold, wherein, when the mechanical brake is operated, the slewing control unit outputs the slewing command until the time period measured by the time measurement unit exceeds a predetermined reference time period, and stops outputting the slewing command after the measured time period exceeds the predetermined reference time period.