A novel copolymer useful as an active ingredient for a positive type resist material to be decomposed by electron beam or X-rays with a high sensitivity and having a superior resistance to dry etching, a process for producing the copolymer and a resist material containing the copolymer are provided, which copolymer consists of 1 to 50% by mol of structural units of ##STR1## and 50 to 99% by mol of structural units of one or more vinyl compounds at least one of which is an aromatic ring-containing vinyl compound and having a number average molecular weight of 500 to 500,000.