Fastener

A fastener comprising:

• a fitted portion fitted into a through hole disposed in an object to be fastened,
• the fitted portion comprising:
• a base portion;
• and
• an elastic piece disposed on a side of the base portion, the elastic piece being configured to allow insertion of the fitted portion into the through hole by elastic deformation of the elastic piece in a direction of approaching the base portion and to engage with an edge of the through hole at an insertion point side of the through hole by elastic return of the elastic piece, the elastic piece being configured to, upon application of force in a direction of pulling out the fitted portion from the through hole, cause an elastic deformation in the direction of approaching the base portion,
• the base portion being provided with an abutting portion abutting on the elastic piece upon application of the force in a direction of pulling out the fitted portion from the through hole, and
• with the force in a direction of pulling out the fitted portion from the through hole being equal to or greater than a predetermined magnitude, the elastic piece bending in a direction toward a central axis of the fitted portion, the central axis being on an inner side relative to the abutting portion, wherein
• the fitted portion includes a guide portion formed to align with one imaginary plane containing the central axis, and
• an end of the fitted portion on the insertion point side of the through hole is formed of an end of the guide portion.