Here, the ammonium fluoride compound is the one represented by the general formula, ##STR1## wherein R6, R7, R8 and R9 is, independently from each other, are a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, such as primary, secondary, tertiary or quaternary ammonium salt in which anions are fluoride ions and a substituent constituting an ammonium salt is a