The present invention provides substrate 10, capable of transmitting lights in thickness direction for enhancing usefulness of the substrate. Its object is to provide a substrate adapted for interconnecting optical elements, allowing reduction of number of components leading to miniaturization and low cost, and high-efficient and stable optical coupling leading to low electric power dissipation. To accomplish the object the substrate comprises substrate body, wherein it is comprised of electric wiring layer disposed on first substrate body surface in such a manner as to be capable of electrically connecting first optical element mounted on the side of first substrate body surface; and optical transmission path, propagating lights being transmitted and received between first optical element electrically connected at electric wiring layer and second optical element disposed on the side of second substrate body surface different from first substrate body surface in substrate body.