To provide a fastener for which load resistance is increased and a retaining property is improved, in the fastener of a type for which the gap of a groove whose cross section is U shaped is narrow and lower ends of a first vertical side part and a second vertical side part are directly connected to each other.

A first contact part 17 which is the connection part of a first horizontal side part 14 and a second horizontal side part 15 can be brought into contact with the inner surface of the first vertical side part 12, and a second contact part 18 which is the connection part of the second horizontal side part and a third horizontal side part 16 can be brought into contact with the inner surface of the second vertical side part 13. Near a position at which the second contact part is brought into contact with the inner surface of the second vertical side part, a projected wall part 20 projected toward the outer side at the second vertical side part is provided.