To facilitate the attachment and detachment of an insert to and from a form and to positively fix the insert to a set boss so as not to come off easily by an impact in placing concrete or vibration in compaction.

Threads are formed at the outer periphery of a boss of the set boss and the inner periphery of an insert opening. When pressing the insert to the boss while rotating, the threads thereby work as guides to facilitate setting. When removing the insert from the form, the boss is forcibly pulled out of the insert using the elasticity of the insert without rotating the boss. Consequently, when the form is separated from the concrete face, the boss comes off from the insert at the same time. Workability is thereby excellent.