To simplify the structure of a buckle by reducing the number of part items and protect a shoulder belt from slipping out during the use.

A buckle comprises a buckle main body 4 arranged at the top end of a crotch belt and a pair male buckles of the right and the left arranged at the top of a pair of waste belt and capable of engaging as freely demountable to the buckle main body 4. The buckle main body 4 has an opening 41 for inserting a male buckle 5 and a window hole 42 communicating thereto. Each of the male buckles 5 has an elastic and deformable tongue part 52 extended from the center end of a base 51 and a guide part 53 extended from the both end of base 51, a demountable shoulder belt fitting member 6 having a fitting hole 6a for fitting the edge of a pair of a right and left shoulder belts 12. On the top end of the tongue 52, an operating part 54 for operating the buckle and a hooking part 55 for hooking the male buckle 5 to the side face 42a of the window part 42 on fitting of the buckle.