An interconnect substrate includes a core layer including a resin layer mainly composed of a non-photosensitive thermosetting resin and a through interconnect extending through the resin layer, the core layer having no reinforcement member contained therein, a first interconnect structure laminated on a first side of the core layer and including first interconnect layers and first insulating layers mainly composed of a photosensitive resin, and a second interconnect structure laminated on a second side of the core layer and including second interconnect layers and a single second insulating layer mainly composed of a photosensitive resin, wherein the first interconnect layers are electrically connected to the second interconnect layers via the through interconnect, wherein the core layer has greater rigidity than the first interconnect structure and the second interconnect structure, and wherein a thickness of the second interconnect structure is greater than a thickness of each of the first insulating layer.