Provided is 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 includes 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; and a resin varnish, a prepreg, and a metal-clad laminate provided using the same.