A balanced-unbalanced conversion circuit includes a first coupling line, an unbalanced terminal connected to the first coupling line, a ground terminal connected to the unbalanced terminal through the first coupling line, a second coupling line electromagnetically coupled to the first coupling line, a first balanced terminal connected to the second coupling line, a second balanced terminal connected to the first balanced terminal through the second coupling line, and a band-reject filter serially connected to the first coupling line to remove predetermined-band signals of high frequency signals transmitted through the first coupling line.