A process for preparing a polyimide which comprises reacting an acid dianhydride represented by the formula: ##STR45## wherein Ar is a divalent group represented by the following formula (a) or (b): ##STR46## wherein hydrogen atoms in each benzene ring is unsubstituted or is substituted with one or more alkyl groups having 1 to 3 carbon atoms or alkoxy groups having 1 to 2 carbon atoms, two or mor