A coupler includes: a hook portion； a stem portion extending from an end portion of the hook portion in a first direction； a slide bar closing and opening the hook portion by reciprocating in the first direction； a connection ring rotatably connected to an end of the stem portion with respect to the first direction, having a predetermined contour, and extending in a second direction perpendicular to the first direction； at least one projection extending from an outer circumferential portion of the connection ring in the second direction by a predetermined length； and an operation member connected to the slide bar and extending toward the connection ring. The operation member abuts only the projection when being moved in the first direction.
