A resin multilayer substrate includes a plurality of insulating resin base material layers and a plurality of conductor patterns provided on the plurality of insulating resin base material layers. The plurality of conductor patterns include a signal line and a ground conductor overlapping the signal line as viewed from a laminating direction of the insulating resin base material layers. A plurality of openings are provided in the ground conductor, and an aperture ratio is higher in a zone far from the signal line than in a zone adjacent to or in a vicinity of the signal line in a direction perpendicular or substantially perpendicular to the laminating direction.