The high-frequency substrate is constructed of a base member and a conductor adhered to the base member, and the base member is composed of a polymer which is a fluoropolymer having a conductor-affinitive monomer graftpolymerized at a grafting percentage of 1% or less by weight. After reactive sites necessary for graftpolymerization are formed on a film of a fluoropolymer under an oxygen-free atmosphere by irradiating the film with an electron beam or the like, the fluoropolymer film is introduced into a solution of a conductor-affinitive monomer so as to cause graftpolymerization, and a conductor is adhered thereto to thereby produce a substrate, with a grafting percentage of the monomer to the fluoropolymer being 1% or less by weight in the graftpolymerization. The technique leads to the production of a high-frequency substrate having superior adhesion force with respect to the conductor.