To enhance the convergency of ions of same m/z, when emitting pulse-likely the ions in an ion source by an EI method.

A gate electrode 15 is arranged between a filament 12 for generation a thermoelectron and a thermoelectron introducing port 102 of an ionization chamber 10. The thermoelectron is shielded by impressing a prescribed voltage to the gate electrode 15, after a sample molecule is ionized by introducing the thermoelectron into the ionization chamber 10. An ion emission voltage is impressed to a repeller electrode 14 after the thermoelectron introduced into the ionization chamber 10 just before is passed through the ionization chamber 10, and initial kinetic energy is imparted to the ions in the ionization chamber 10, to emit the ions pulse-likely from the ionization chamber 10. An electric field is not disturbed at that time by the thermoelectron, and a behavior of the ion is not affected by the thermoelectron, because no thermoelectron exists substantially in the ionization chamber 10. A noise is also restrained from being generated by collision of the thermoelectron with the repeller electrode 14.