A fastener, comprising an expander part having a saucer-shaped head and a shank extending from the head and formed with a male thread thereon, and a fastener part having a flange and opposite stem portions branched from the back of the flange and connecting through hinge portions to guide portions which join each other at the other ends thereof, the expander part and fastener part each being made of a resin, the fastener part having a circular bore extending from the flange to the stem portions to permit the expander part to be inserted in and temporarily retained in the stem portions, the circular bore having therein opposite bulges positioned ahead of the hinge portions to spread the stem portions outward away from each other when the expander part is pushed.