A wiring substrate includes a substrate body formed of a plate-like ceramic, having a front surface, a back surface, and a height of 0.8 mm or less; a cavity opening at the front surface and having a rectangular shape as viewed in plane; and side walls having a thickness of 0.3 mm or less between a side surface of the cavity and a side surface of the substrate body. The wiring substrate further includes an electrically conductive layer having the form of a frame and formed on the front surface to surround an opening of the cavity; a ceramic surface having the form of a frame and located adjacently to the electrically conductive layer and along the outer periphery of the front surface; and a via conductor formed in the substrate body along the side surface of the cavity between a bottom surface of the cavity and the front surface.