The buckle of the present invention is a buckle, of which engagement claw can be engaged with a serrate engagement groove of a band. The buckle includes: a buckle main body composed of a base plate and a roof frame, and two side plates connecting the base plate and the roof frame; and an operation piece being installed inside the roof frame, and having the engagement claw that can be engaged with the engagement groove of the band. The operation piece is movably supported by a connecting shaft connecting the two side plates in an intermediate point. The buckle main body is provided with a control piece for controlling a swing of the operation piece. The engagement claw is engaged with the engagement groove of the band, and the control piece functions as a spring for biasing the engagement claw toward the engagement groove side in such a locked state.