Buckle includes a male member having a pair of arms each having an engaged part, and a flat cylinder-shaped female member having an insertion port into which the pair of arms is inserted and an engaging part engaging the pair of arms. The female member includes a first cylinder side wall and a second cylinder side wall, and an opened port opposed to the insertion port. The pair of arms is not exposed from an edge of the opened port in the state where the engaging part is engaged with the engaged parts, when viewed from a plane opposed to an outer surface of the first cylinder side wall. The pair of arms is exposed from the opened port in the state where the engaging part is engaged with the engaged parts, when viewed from a plane opposed to an outer surface of the second cylinder side wall.