An interposer substrate includes a body, first to third external connection conductors, and a wiring conductor. The body includes first to third principal surfaces. A distance between the first and second principal surfaces is different from a distance between the first and third principal surfaces. The first external connection conductor is provided on the first principal surface and is connected to an external circuit board. The second external connection conductor is provided on the second principal surface and is connected to a first flat cable. The third external connection conductor is provided on the third principal surface and is connected to a second flat cable. The wiring conductor is provided in the body, and connects the first external connection conductor and second and third external connection conductors.