A buckle having a buckle main body and an insertion body. The buckle main body comprises an upper plate, a lower plate, and side walls. The buckle main body also has an insertion port at one end and a belt-mounting portion at the other end. The insertion body has flexible operation rods on both sides of a base rod in a projecting manner where portions of the operation rods are to be engaged in the insertion port of the buckle main body. The operation rods have narrow leg portions arranged in a side of the base rod, and bulging head portions fitted to opening portions and formed in the front end of the leg portions. Reinforcing ridge portions are formed on the outer surface of the leg portions running from substantially the base rod to substantially the bulging head portions.