The present invention provides a semiconductor element mounting substrate 101 including: a base substrate 1 having a region 2 for mounting a semiconductor element 11, the region 2 being set on the major surface of the base substrate 1; a plurality of wiring patterns 3 formed on the base substrate 1 and connected to the semiconductor element 11; and a dummy pattern 8 formed like a frame in the region 2 for mounting the semiconductor element 11 and not connected to the wiring patterns 3.