The present invention aims to provide a substrate for mounting an IC chip, on which an optical signal passing region is formed and which can suppress a transmission loss in an optical signal and transmit an optical signal more positively with high reliability. The substrate for mounting an IC chip according to the present invention is a substrate for mounting an IC chip, in which a conductor circuit and an insulating layer are laminated in alternate fashion and in repetition on both faces of a substrate and an optical element is mounted on the substrate. Herein, the substrate for mounting an IC chip includes an optical signal passing region, and a microlens arranged on an end portion of the optical signal passing region on the opposite side from the optical element.