A tensioning device is provided to realize compactness and simplification of device structure and causes mountability to be improved. A tensioning mechanism to rapidly tension one end of an interconnecting member when deceleration of a vehicle is applied, a reversing-prevention hook plate supported rotatably freely on an arm plate, a buckle joined freely rotatably at a position most proximate to a seat belt in a direction of tension action of the seat belt within a plate surface of the reversing-prevention hook plate, and a reversing-prevention claw to restrict reverse rotation of the reversing-prevention hook plate after the interconnecting member has been pulled out are provided, and a base end of the foregoing arm plate is joined freely rotatably to a frame member. During tensioning operation, the arm plate rotates in the direction of tensioning of the interconnecting member, and along with this the reversing-prevention hook plate rotates in a direction opposite the direction of rotation of the arm plate while pulling out the interconnecting member.