The present invention provides a block copolymer comprising at least one segment having an acid group and at least one segment substantially free from an acid group, wherein the segment having an acid group comprises a repeating unit which is a substituted repeating unit represented in the formula (1) with an acid group, --(Ar.sup.1--X.sup.1--Ar.sup.2--X.sup.2)--, (1) and in the formula(1), X.sup.1 and X.sup.2 being each independently --O-- or --S--, Ar.sup.1 and Ar.sup.2 being each independently an aromatic group selected from the groups represented by the following formulae (2) to (4) ##STR00001##