To provide a substrate for mounting a semiconductor element, in which fine-pitch wiring layers are formed to allow a semiconductor element to be mounted, while heat generated in the semiconductor element will not result in a decrease in reliability. Semiconductor-element mounting substrate sandwiches low-thermal-expansion substrate with upper interlayer resin layer and lower interlayer resin layer, and conductive circuit of organic substrate and first conductive circuit of low-thermal-expansion substrate are connected by via conductor formed in interlayer resin layer. Therefore, low-thermal-expansion substrate for mounting semiconductor element may be connected to organic substrate that is connected to outside substrates, without arranging an organic substrate and resin layers on the lower surface of low-thermal-expansion substrate, where impact from the thermal history of semiconductor element is notable.