Radio wave transmissive substrate

A radio-wave transmitting substrate includes a dielectric substrate, and, on at least one main surface of the dielectric substrate, a heat-ray reflection film including an electroconductive film, and an opening where the electroconductive film is absent in a plan view. At least a part of the at least one main surface in a plan view is a radio-wave transmitting region. The radio-wave transmitting region is a region where every 1-cm square unit region in the region satisfies the following expression (a): L>802.6×S−503.7. L is an overall length (unit: mm/cm2) of a boundary between the heat-ray reflection film and the opening in the unit region, and S is a proportion of an area occupied by the heat-ray reflection film in the unit region.