A wiring board comprising: a board; two wirings (L1, L2; L3, L4) disposed on the board in parallel; an insulation resin layer which is formed on part of a face of the board, wherein 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; wherein the two wirings (L1, L2; L3, L4) extend from the face of the board to the top face of the insulation resin layer so as to traverse the stepped portion; characterized in that: the two wirings (L1, L2; L3, L4)constitute a differential transmission line (P1; P2); the extending direction of the two wirings (L1, L2; L3, L4) 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 constituting the stepped portion; and wherein the two wirings (L1, L2; L3, L4) connect two terminal pads (16A, 16B; 16C, 16D) disposed on the board with a first pitch to two terminal pads (12A, 12B; 12C, 12D) disposed on the insulation resin layer with second pitch, where the second pitch is greater than the first pitch. a board; two wirings (L1, L2; L3, L4) disposed on the board in parallel; an insulation resin layer which is formed on part of a face of the board, wherein 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; wherein the two wirings (L1, L2; L3, L4) extend from the face of the board to the top face of the insulation resin layer so as to traverse the stepped portion; characterized in that: the two wirings (L1, L2; L3, L4)constitute a differential transmission line (P1; P2); the extending direction of the two wirings (L1, L2; L3, L4) 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 constituting the stepped portion; and wherein the two wirings (L1, L2; L3, L4) connect two terminal pads (16A, 16B; 16C, 16D) disposed on the board with a first pitch to two terminal pads (12A, 12B; 12C, 12D) disposed on the insulation resin layer with second pitch, where the second pitch is greater than the first pitch. the two wirings (L1, L2; L3, L4)constitute a differential transmission line (P1; P2); the extending direction of the two wirings (L1, L2; L3, L4) 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 constituting the stepped portion; and wherein the two wirings (L1, L2; L3, L4) connect two terminal pads (16A, 16B; 16C, 16D) disposed on the board with a first pitch to two terminal pads (12A, 12B; 12C, 12D) disposed on the insulation resin layer with second pitch, where the second pitch is greater than the first pitch.