Wiring board comprising three or more insulating layers, where a post wall waveguide of staircase shape is formed therein

A wiring board comprising:

• a built-in post wall waveguide having a region, surrounded by two mutually opposing conductors and a first post wall and a second post wall connecting the two conductors, and serving as a transmission path for electromagnetic waves, wherein:
• the two conductors are disposed to oppose each other with three or more insulating layers interposed therebetween,
• the first post wall has a configuration including first columnar portions, formed by a laminate of via interconnects penetrating the three or more insulating layers, arranged at predetermined intervals in a first direction in which the electromagnetic waves are transmitted,
• the second post wall has a configuration including second columnar portions, formed by a laminate of via interconnects penetrating the three or more insulating layers, arranged at predetermined intervals in the first direction,
• in a cross sectional view taken in a second direction perpendicular to the first direction, the via interconnects forming the first columnar portion and the second columnar portion are laminated in a staircase shape, and an interval between adjacent first via interconnects opposing each other in one of the three or more insulating layers not in contact with one of the two conductors is wider than an interval between adjacent second via interconnects opposing each other in two of the three or more insulating layers in contact with the two conductors, respectively, and
• in the cross sectional view, each of the adjacent first via interconnects has a width that gradually narrows from wide portions located at both mutually opposite surfaces of the one of the three or more insulating layers toward a narrow portion located at a center along a thickness direction.