Provided is a fastener, which has advantages over the prior art in that a retaining force is not dependent upon the biasing force of a spring, the fastener hardly comes out, and is less restricted in size than the prior art. The fastener has another important advantage over the prior art in that it forms no hole in plain cloth. A fitting portion (2) and a base portion (3) are assembled to have a clamping unit (1) opened. A stopper (4) is pushed together with the plain cloth into the inside of the clamping unit (1). This clamping unit (1) is closed to hold the plain cloth between the outer circumference of the stopper (4) and the leading ends of holding means (26 and 36). With slide portions (22) being fully fitted in groove portions (32), retaining pawls (23) are fitted in pawl receivers (33b).