Provided is a process for producing a thermosetting resin varnish from which a printed circuit board can be produced wherein the printed circuit board has excellent dielectric properties in a high frequency band and can significantly reduce the transmission loss, and exhibits excellent heat resistance after moisture absorption and excellent thermal expansion properties and satisfies the metallic foil peeling strength.A process for producing a thermosetting resin varnish containing a thermosetting resin composition, which contains an uncured semi-IPN composite, an inorganic filler, and a saturated thermoplastic elastomer, wherein the process comprises the steps of: (i) preliminary reacting (B) a butadiene polymer which contains in the molecule thereof 40% or more of a 1,2-butadiene unit having a 1,2-vinyl group in the side chain thereof, and which is not chemically modified and (C) a crosslinking agent, in the presence of (A) a polyphenylene ether to obtain a polyphenylene ether-modified butadiene prepolymer which is an uncured semi-IPN composite; (ii) mixing together (D) an inorganic filler and (E) a saturated thermoplastic elastomer to obtain a mixture; and (iii) mixing together the obtained mixture and the polyphenylene ether-modified butadiene prepolymer, a resin varnish, a prepreg, and a metal-clad laminate provided using the same.