1. A substrate for a high-frequency printed wiring board, the substrate comprising:a dielectric layer including a fluororesin and an inorganic filler; and
a copper foil layered on at least one surface of the dielectric layer, wherein
a surface of the copper foil at the dielectric layer side has a maximum height roughness (Rz) of less than or equal to 2 ?m,
a ratio of the number of inorganic atoms of the inorganic filler to the number of fluorine atoms of the fluororesin in a superficial region of the dielectric layer at the copper foil side is less than or equal to 0.08, and
a softening temperature of the fluororesin is more than or equal to 250° C. and less than or equal to 310° C.