To make a seat belt exchangeable as necessary and also to expand applicability.

A seat belt 11 has a belt member 21 that restrains a seated infant on a concave seat part 52 of a chair 51. In the seat belt, a flat fixing member 31 is held in a terminal part to be fixed in a belt member 21 relative to the surface of the chair 51, with a projected part 34 erected on one side of the fixing member 31. Meanwhile, the chair 51 is formed by molding of expandable resin and provided with softness and flexibility. The projected part 34 is sharply formed and pierced from the surface of the concave seat part 52, with a fastening member 41 latched to the tip end side portion of the projected part 34 that sticks out in the reverse side, thereby maintaining the fixing member 31 in a fixed state in the configuration of the chair.