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