A buckle for tightening or loosening a band, including: a buckle body having a pair of hinge holes; a plate spring arranged in the buckle body with a leading part positioned in a sliding path of the band, the leading part being shaped to slope with a leading end contacting a back side of the band so as to be caught by a catch projection of the band; and a buckle switch having a pivot pressure piece with hinge projections, and an operation lever connected to a pivot pressure piece so as to be exposed to one side of the buckle body. The leading end of the plate spring is separated from the band so as to make the band slide in a loosening direction when the plate spring is pressed by the pivot pressure piece.