Provided is a thermosetting resin composition which can be used for the production of printed circuit boards, having good dielectric properties in high frequency bands so that transmission loss can be significantly lowered, having excellent heat resistance after moisture absorption and thermal expansion properties, and satisfying peeling strength between the resin composition and metal foil.The present invention relates to a thermosetting resin composition of a semi-IPN composite, comprising (A) a polyphenylene ether, and a prepolymer formed from (B) a chemically unmodified butadiene polymer containing 40% or more of a 1,2-butadiene unit having a 1 2,-vinyl group in a side chain of a molecule and (C) a crosslinking agent, in a compatibilized and uncured state; and a resin varnish, a prepreg and a metal clad laminated board using the same.