In a positive resist composition comprising (A) a resin component which becomes soluble in an alkaline developer under the action of an acid and (B) a photoacid generator, component (A) is a polymer of formula (1) wherein R1 is H, methyl or trifluoromethyl, R2 and R3 are alkyl, R4 is a monovalent hydrocarbon group, X1 is O, S or CH2CH2, X2 is O, S, CH2 or CH2CH2, n is 1 or 2, a and b each are from 0.01 to less than 1, c, d1 and d2 each are from 0 to less than 1, and a+b+c+d1+d2=1. The resist composition forms a pattern with high rectangularity at an enhanced resolution when processed by ArF lithography.