o 60% of the hydroxy groups by tert-butoxycarbonyloxy groups and a second poly(hydroxystyrene) resin substituted for from 10% to 60% of the hydroxy groups by the groups represented by the general formula --O--CR1 R2 (OR3), in which R1 is a hydrogen atom or a methyl group, R2 is a methyl group or ethyl group, and R3 is a lower alkyl group having 1 to 4 carbon atoms, in a weight proportion in the ra