A mixture of sulphamic acid, a halogenated sulphonic acid and thionyl chloride is heated to allow the reaction to proceed, to thereby produce first intermediate products. The first intermediate products are then subjected to reaction with an alkali metal fluoride MF to produce second intermediate products. The second intermediate products is then subjected to reaction with the alkali metal fluoride MF in a polar solvent to obtain a desired product MN(SO2F)2 (where M is an alkali metal).