To provide a fastener capable of showing effects for prevention of separation and looseness prevention after execution of works by combining a sleeve and a plug in an inlaid state before and after execution of works and releasing it with one action at a releasing process.

Before execution of works, the sleeve 3 and the plug 4 pressed into the sleeve are positioned at an inlaid combination between a fitting projection 8 in an inner periphery of an expansion portion 7 and a neck 11 at a plug 4 side. After execution of works, the fitting projection 8 in the inner periphery of the expansion portion 7 is again combined with a neck 10 at the plug 4 side in an inlaid state, and the looseness preventive effect on vibration or the like is shown for both of them. When a fastener is released, expansion state of the fastener is released with one action by giving knocking force on the pointed head of the plug 4, and the fastener 1 can be immediately pulled out.