The present invention is a polymer obtained by using a silsesquioxane derivative represented by formula (1-0). In the formula R.sup.0 each independently represents hydrogen, alkyl, aryl or arylalkyl, and Y each represents a group represented by formula (a) or formula (b). In each of formula (a) and formula (b), at least one of X represents hydrogen, chlorine or a group having one of --CH.dbd.CH--, --C.ident.C--, --OH, --COOH, --COO--, 2-oxapropane-1,3-dioyl, oxiranyl, oxiranylene, oxetanyl, oxetanylene, 3,4-epoxycyclohexyl, --SH, --NH-- and --NH.sub.2, and in formula (b), Z represents --O--, --CH.sub.2-- or a single bond. ##STR00001##