A tensioner and a method of controlling tension for a three-point seat belt installed in a vehicle include an adjuster that varies tension of the three-point seat belt, a detector that signals a direction of a turn made by the vehicle, and a controller that is coupled to the adjuster and to the detector. The three-point seat belt is used to restrain both sides of an occupant's lumbar region as well as one of the occupant's right and left shoulders. The controller controls the adjuster so that the tension of the three-point seat belt when the direction of the turn is opposite to the one of the occupant's right and left shoulders that is restrained by the three-point seat belt is set lower than when the direction of the turn corresponds to the one of the occupant's right and left shoulders that is restrained by the three-point seat belt.