PURPOSE: To improve assembling properties without impairing the riding feeling and seat belt operation, etc. by folding back the seat belt at a belt guide and by winding it into a retractor through a gap formed on the same side along one side of a cushion frame.

CONSTITUTION: An arresting device is constituted so that a seat belt 6 is guided through a belt guide 7 and is wound by a retractor 5 arranged on the front end inside of a cushion frame 11 through a guide roller 10. The retractor 5 is fitted to the front assembly section (a) of a seat adjuster 4 and one movable rail 12a through a casing 13, and the guide roller 10 is pivoted on it. A gap 14 is formed on one side 11a of a frame at the place corresponding to the fitting section of the retractor 5 from above to the side, then a retractor casing 13 is arranged inside of it and corresponding to it so that the guide roller 10 faces the gap 14.