Disclosed is a process for producing a photoresist polymeric compound. The process includes the steps of polymerizing a monomer mixture containing at least one monomer selected from a monomer (a) containing a group capable of leaving with an acid to allow the polymeric compound to be soluble in an alkali, a monomer (b) having a lactone skeleton, and a monomer (c) having a hydroxyl-containing alicyclic skeleton, to give a polymer; passing a solution containing the polymer through a filter including a porous membrane having an anion-exchange group to give a polymer solution; and thereafter passing the polymer solution through a filter including a porous membrane having a cation-exchange group. The polymer solution before passing through the filter including a porous membrane having a cation-exchange group preferably has a content of metals of 1000 ppb by weight or less per the weight of the polymer.