A semiconductor mounting device including a first substrate having first insulation layers, first conductor layers formed on the first insulation layers and via conductors connecting the first conductor layers, a second substrate having a core substrate, second conductor layers, through-hole conductors and buildup layers having second insulation layers and third conductor layers, first bumps connecting the first and second substrates and formed on the outermost first conductor layer on the outermost first insulation layer, and second bumps positioned to connect a semiconductor element and formed on the outermost third conductor layer on the outermost second insulation layer. The second substrate has greater thickness than the first substrate, the second conductor layers are formed on surfaces of the core substrate, respectively, the through-hole conductors are formed through the core substrate and connecting the second conductor layers, and the buildup layers are formed on the core substrate and second conductor layers, respectively.