A buckle including a male member; and a female member into which the male member can be inserted for engagement, when an external force in a direction opposite to the insertion direction is exerted on the male member after an engaging face of the male member and an engaged face of the female member being engaged, an engagement width between the engaging face and the engaged face becomes larger than an engagement width before the external force being exerted due to the elastic deformation of legs of the male member exposed through openings in an opposite direction to a side on which the legs faces each other.