To allow an occupant to easily wear a seat belt without twisting his or her body when the occupant sits in a seat.When a door sensor 60 detects the opening of a door, a motor is driven to wind up a seat belt 2 so as to move a belt guide 4 against biasing force of a spring 9, thereby evacuating the seat belt 2 to a rear side of a vehicle relative to a seat back 1a. When the door sensor 60 detects the closing of the door and a seat sensor 61 detects the sitting of the occupant, the motor is driven in the reverse direction to make a clutch mechanism 34 in the disconnected state so as to protrude the seat belt 2 in the forward direction of the vehicle by the biasing force of the spring 9.