Production method of porous resin

A production method of a porous resin, 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, and wherein, in the suspension copolymerization, the proportion of the amount of the acyloxystyrene monomer in the total amount of monomer is 0.2–20 wt %, and the proportion of the amount of the divinylbenzene monomer in the total amount of the monomer is 2–30 wt %.