A belt mounting structure in a buckle which can securely fix a belt in use and allows easy adjustment of an effective belt length and removal of the belt. A belt winding rod freely reciprocates in a direction of fastening the belt and freely rotates at a predetermined angular range within a belt mounting hole. A bulging portion is formed along an upper end edge of the belt mounting hole in the side to which the buckle is connected. A cross-section of a main body of the belt winding rod is formed substantially in a waterdrop shape. A cross-section of each supporting axial portion of the belt winding rod is formed substantially in a shape of an isosceles triangle. When an oblique side portion of the axial portion is mounted and supported on a long side portion of a supporting hole, a tip portion of the main body is pressure contacted to a lower surface of the bulging portion via the belt. When the axial portions rotate to the side of an upper surface of the buckle within the supporting holes, the tip portion of the main body can rotate in the same direction over the bulging portion with the belt while avoiding an interference with the bulging portion.