A fastener provided with a pin and a body configured to insert the pin. The pin has a pin flange and a pin shaft extending in the vertical direction from the pin flange; and the main body has a bottom, an opening formed in the bottom for inserting the pin, a member to be mounted locking part formed vertically from the bottom along the opening in order to lock to the member to be mounted, a pin flange temporary holding part for temporarily fixating, before attaching to the member to be mounted, the pin flange at a first position opposite to the member to be mounted locking part relative to the bottom, and a pin flange lock for locking the pin flange at a second position closer to the bottom than the first position while mounted on the member to be mounted.