A mooring line tension control system (1) includes: a drum (3) around which a mooring line is wound; and control circuitry (7) that controls a motor (22), a band brake (4), and a clutch (24). The motor (22) rotates the drum (3). A load cell (40) that measures band tension is incorporated in the band brake (4). In a ship mooring state, the control circuitry (7) calculates tension of the mooring line based on the band tension measured by the load cell (40), and in a case where the calculated tension of the mooring line is greater than a setting value, the control circuitry (7) switches the band brake 4 to a released state to unwind the mooring line from the drum (3), and thereafter switches the clutch (24) to an engaged state and drives the motor (22) to wind the mooring line around the drum (3).