The present invention relates to a substrate having a dielectric loss tangent (A) as measured at 20° C. and 10 GHz of 0.1 or less, a dielectric loss tangent (B) as measured at 20° C. and 35 GHz of 0.1 or less, and a ratio [a dielectric loss tangent (C) as measured at an arbitrary temperature in a range of −40 to 150° C. and at 10 GHz]/[the dielectric loss tangent (A)] of 0.90-1.10, or a substrate having a relative permittivity (a) as measured at 20° C. and 10 GHz of 4 or more and 10 or less, a relative permittivity (b) as measured at 20° C. and 35 GHz of 4 or more and 10 or less, and a ratio [a relative permittivity (c) as measured at an arbitrary temperature in a range of −40 to 150° C. and at 10 GHz]/[the relative permittivity (a)] of 0.993-1.007.