A buckle, comprising: a male member including a pair of arms each having an engaged part near a front end; and a flat cylinder-shaped female member including, at one cylinder end, an insertion port into which the pair of arms is inserted, and an engaging part, at the other cylinder end, engaging the pair of arms in a cylinder by engagement with the engaged parts, wherein the female member includes a first cylinder side wall and a second cylinder side wall, extending in a flattened direction and being opposed to each other, and an opened port opposed to the insertion port at the other cylinder end; the second cylinder side wall is shaped such that at least one of the pair of arms is exposed from the opened port defined by the second cylinder side wall 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; wherein the engaged parts include second engaged protrusions protruding toward the second cylinder side wall from the pair of arms, and a width of the second engaged protrusions in a thickness direction of the second cylinder side wall is smaller than a width of the second cylinder side wall; and wherein the male member further includes first engaged protrusions protruding toward the first cylinder side wall from the pair of arms, and is shaped to be symmetric about a plane including the pair of arms; and the female member has a front side guide groove that extends from the insertion port to the opened port and guides the front side protrusions between the insertion port and the opened port, and a back side guide groove that guides the second engaged protrusions, wherein the second cylinder side wall has a second opening edge as the engaging part constituting an edge of the opened port, which engages with either the first engaged protrusions or the second engaged protrusions characterized by that the first cylinder side wall is shaped such that the pair of arms is not exposed from an edge of the opened port defined by the first cylinder side wall in a 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. a male member including a pair of arms each having an engaged part near a front end; and a flat cylinder-shaped female member including, at one cylinder end, an insertion port into which the pair of arms is inserted, and an engaging part, at the other cylinder end, engaging the pair of arms in a cylinder by engagement with the engaged parts, wherein the female member includes a first cylinder side wall and a second cylinder side wall, extending in a flattened direction and being opposed to each other, and an opened port opposed to the insertion port at the other cylinder end; the second cylinder side wall is shaped such that at least one of the pair of arms is exposed from the opened port defined by the second cylinder side wall 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; wherein the engaged parts include second engaged protrusions protruding toward the second cylinder side wall from the pair of arms, and a width of the second engaged protrusions in a thickness direction of the second cylinder side wall is smaller than a width of the second cylinder side wall; and wherein the male member further includes first engaged protrusions protruding toward the first cylinder side wall from the pair of arms, and is shaped to be symmetric about a plane including the pair of arms; and the female member has a front side guide groove that extends from the insertion port to the opened port and guides the front side protrusions between the insertion port and the opened port, and a back side guide groove that guides the second engaged protrusions, wherein the second cylinder side wall has a second opening edge as the engaging part constituting an edge of the opened port, which engages with either the first engaged protrusions or the second engaged protrusions characterized by that the first cylinder side wall is shaped such that the pair of arms is not exposed from an edge of the opened port defined by the first cylinder side wall in a 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.