Disclosed is a substrate for a semiconductor package in which leakage of radiation noise from a gap between a semiconductor element and a mounting substrate can be prevented. The substrate for the semiconductor package includes a coplanar waveguide including a signal and ground electrodes on the mounting substrate, the signal electrode flip-chip connected to the semiconductor element, the ground electrodes arranged on both sides of the signal electrode with intervals therebetween. A step part is formed in the ground electrodes in an outer circumferential part of a mounting region of the semiconductor element, the step part having a larger distance between upper surfaces of the mounting substrate and the ground electrode in the outer circumferential part of the mounting region than such distance in the mounting region, and an insulator for covering the signal electrode in the outer circumferential part of the mounting region is formed.