A buckle is formed of a base having two side walls, a cover for accommodating and supporting the base, and having a tongue insertion hole formed at one end side of the two side walls of the base, and guide portions provided on each of the two side walls to protrude inwardly therefrom for leading a tongue to be inserted through the tongue insertion hole. The guide portions lead a tip of the tongue smoothly without blocking even when the tongue is inserted from the tongue insertion hole at a maximum slant posture defined by the tongue insertion hole relative to the buckle. The tongue can be smoothly inserted into the buckle to improve the manipulation feeling in inserting the tongue.