A mount structure includes a wiring board and a semiconductor device composed of a light-emitting device or a light-receiving device mounted on one surface side of the wiring board such that an optical axis thereof is oriented in a direction that extends along a board surface of the wiring board. On the one surface side of the wiring board, a first pad on which a first terminal of the semiconductor device is mounted, a second pad on which a second terminal of the semiconductor device is mounted, and a light-shielding conductive layer are formed using the same conductive layer. The first pad and the second pad are arranged on respective sides of an imaginary center line along which the optical axis of the semiconductor device extends and the light-shielding conductive layer is provided at a position beneath a light emission center or a light reception center of the semiconductor device in plan view.