Provided is a wiring board including: a board; a differential transmission line constituted by two wirings disposed on the board in parallel; an insulation resin layer which is formed on part of a face of the board. A stepped portion constituted by a lateral face of the insulation resin layer is formed at a boundary between the face of the board and a top face of the insulation resin layer. The two wirings extend from the face of the board to the top face of the insulation resin layer so as to traverse the stepped portion. The extending direction of the wirings traversing the stepped portion and the direction of a periphery are perpendicular to each other in a plan view of the board, the periphery being defined by a boundary between the top face of the insulation resin layer and the lateral face of the insulation resin layer.