In case when an external force in a direction of pulling out from a female member acts on a male member in a fully fastened state, the male member moves in an axial direction, and a position where claw parts of engaging leg pieces engage moves from mounting parts and to contact parts. An engaging slot is formed on the contact part, and the engaging slot therefore engages with the claw part of the engaging leg piece. Movement in the direction of rotation of the male member is thereby restricted, and the male member is moved in state along the axial direction, so that the claw parts of the engaging leg pieces do not engage with engaging holes, and a state in which a vehicle body panel and a trim board are sandwiched by a flange part and the engaging leg pieces can be maintained.