A low dielectric substrate for high-speed millimeter-wave communication includes a quartz glass cloth with a dielectric loss tangent of 0.0001 to 0.0015 and a dielectric constant of 3.0 to 3.8 at 10 GHz, and an organic resin with a dielectric loss tangent within 80% to 150% of the dielectric loss tangent of the quartz glass cloth at 10 GHz and a dielectric constant within 50% to 110% of the dielectric constant of the quartz glass cloth at 10 GHz. This provides a low dielectric substrate for high-speed millimeter-wave communication where the low dielectric substrate makes it possible to send signals that are stable and have excellent quality with no difference in propagation time between wirings even if the substrate has an uneven resin distribution and the quartz glass cloth above and below the wirings, and the difference in dielectric loss tangent between members has been reduced to lower transmission loss.