An optical waveguide is produced by using a polymer obtained by using a silsesquioxane derivative represented by the formula (1-0). ##STR00001## In the formula (1-0), R.sup.0 independently represents hydrogen, alkyl having 1 to 40 carbon atoms in which optional hydrogen may be replaced by fluorine and optional --CH.sub.2-- may optionally be replaced by --O--, --CH.dbd.CH--, cycloalkylene, or cycloalkenylene, aryl in which optional hydrogen may optionally be replaced by halogen, or with alkyl having 1 to 20 carbon atoms whereby optional hydrogen may be replaced by fluorine and optional --CH.sub.2-- may be replaced by --O--, --CH.dbd.CH--, cycloalkylene, or phenylene, or arylalkyl in which optional hydrogen in the aryl may be replaced by halogen or alkyl having 1 to 20 carbon atoms, and hydrogen in the alkylene of the arylalkyl may be replaced by fluorine, and optional --CH.sub.2-- in the alkylene of the arylalkyl may be replaced by --O--, --CH.dbd.CH--, or cycloalkylene, and Y is a group represented by the formula (a) or (b): ##STR00002##