A semiconductor element package includes a base body, a frame member, and a terminal member. The frame member is provided on a main surface of the base body. A notch is formed on the base body side of this frame member. The notch is a gap between the one main surface of the base body and the frame member. The terminal member is provided so as to cover the notch as the gap. The terminal member includes a first dielectric layer, a plurality of signal wiring conductors and a plurality of coplanar ground conductor layers that are provided on one surface of the first dielectric layer, and a second dielectric layer. The first dielectric layer has a hole provided open in a region of the one surface between a first wiring conductor and a second wiring conductor.