According to one embodiment, a coupler for transmitting and receiving electromagnetic wave by electromagnetic coupling between the coupler and another, includes a line-shaped coupling element including a first open end and a second open end, a ground plane, a feeding element connecting the coupling element and a feed point, and a short circuiting element connecting the feeding element and the ground plane. The feeding element includes a first end connected to an intermediate portion between the first open end and the second open end of the coupling element, and a second end connected to the feed point. The short circuiting element includes a third end arranged between the first end of the feeding element and the second end of the feeding element, and a fourth end connected to the ground plane.