A fastener in which a sleeve 3 and a plug 4 are put into concavo-convex engagement both before and after execution to exert an effect of preventing separation between the two and an effect of preventing loosening therebetween after execution and which can be released by a single action during disassembling. Before execution, the sleeve 3 and the plug 4 press fitted therein are positioned by concavo-convex engagement between a fitting protrusion 8 on the inner periphery of an expanding portion 7 and a neck portion 11 of the plug 4. After execution, the fitting protrusion 8 on the inner periphery of the expanding portion 7 as expanded put into concavo-convex engagement with a neck portion 10 of the plug 4 again, thereby exerting an effect of stopping loosening between the two with respect to vibration, etc. During disassembling, a striking force is given to an end of the plug 4 to release the expanded state by a single action, allowing immediate pulling-away of the fastener 1.