Among the quaternary ammonium salts (II), preferable ones are compounds wherein R5 and R6 represent each an alkyl or hydroxyalkyl group having 1 or 2 carbon atoms, R7 and R8 represent each an alkylene group having 1 or 2 carbon atoms, Q is a group represented by the formula: ##STR38## or the formula: ##STR39## (wherein R14' represents a hydrogen atom or a methyl group); R9 and R10 represent each a