To reduce connection defects between a circuit substrate provided on a core substrate and a circuit to be mounted thereon, thereby improving reliability as a multilayered device mounting substrate. The device mounting substrate includes: a first circuit substrate composed of a substrate, an insulating layer formed on this substrate, and a first conductive layer (including conductive parts) formed on this insulating layer; and a second circuit substrate mounted on the first circuit substrate, being composed of a base, a second conductive layer (including conductive parts) formed on the bottom of the base, and a third conductive layer (including conductive parts) formed on the top of the base. Here, the first and second circuit substrates are bonded by pressure so that the first and second conductive parts are laminated and embedded together into the insulating layer. The first and second conductive parts form connecting areas in the insulating layer, thereby connecting the first and second circuit substrates electrically.