Provided are a heat dissipation substrate capable of improving heat dissipation properties of an electronic component, and an electric power steering device. In the heat dissipation substrate, a plurality of thermal vias are disposed at least in an electronic component projection region in which a region of a bottom surface portion of the electronic component is projected to a mounting surface in a direction perpendicular to the mounting surface, and a surface density of the thermal vias which occupy the mounting surface per unit area is at least partially different. The plurality of thermal vias are disposed so that the surface density of the thermal vias becomes the greatest in a dense region on an inner side of an edge portion of the electronic component projection region.