A production method of a porous resin or an intermediate therefor, which comprises a step of producing a styrene-acyloxystyrene-divinylbenzene copolymer comprising suspension copolymerization of a styrene monomer, an acyloxystyrene monomer and a divinylbenzene monomer using an organic solvent and water, and a step of converting the styrene-acyloxystyrene-divinylbenzene copolymer to a styrene-hydroxystyrene-divinylbenzene copolymer, wherein a chain transfer agent is added to the suspension copolymerization system.