Abstract: An atmospheric pressure ionization method uses: a gas flow passage control unit (26) and a gas outlet nozzle (24) configured to jet argon gas to an atmospheric atmosphere; a needle electrode (19) arranged between an outlet port of the gas outlet nozzle (24) and an introduction port of an ion introduction pipe (6) that includes a tip end portion formed into a two-sheeted hyperboloid of revolution having a radius of curvature of 1 ?m or more and less than 30 ?m; a needle electrode support mechanism (20); and an electric power generation unit (22) configured to apply a voltage to the needle electrode (19). The atmospheric pressure ionization method includes: applying a voltage of 1.8 kV or more to the needle electrode (19) from the voltage generation unit (22) to generate a dark discharge; exciting the argon gas with the dark current; and causing the excited argon gas and the sample to react with each other, to thereby ionize the sample.