An electronic component mounting substrate including a support layer made of resin with first and second surfaces, an organic insulation layer on the first surface of the support layer with a first surface on opposite side of the first surface of the support layer and a second surface in contact with the first surface of the support layer, an inorganic insulation layer on the first surface of the organic layer, a conductor on the second surface of the support layer, and a first conductive circuit on the second surface of the organic layer. The inorganic layer has a second conductive circuit and a pad for mounting an electronic component inside the inorganic layer. The organic layer has a via conductor inside the organic layer and connecting the first and second circuits. The support layer has a conductive post inside the support layer and connecting the first circuit and the conductor.