To correctly and speedily measure a concentration of helium in a neon gas by determining a sample gas sampled from the neon gas by a mass spectrometer with an acceleration slit which passes only helium ions.

A neon gas injected to a mass analysis pipe which is kept to be vacuum by an evacuation system is ionized in an ion chamber 2 by electron beams from a filament 1. The generated helium ions are drawn out by an acceleration slit 3 and emitted to an external space of the acceleration slit 3. The emitted helium ions fly along a circular orbit determined by a mass, an intensity of a magnetic field 7 and a fly speed. An ion collector 5 is set to meet the orbit where the helium ions fly, so that only helium ions are caught into the ion collector 5. The helium ions are amplified by a current amplifier 6, and a concentration of helium is detected in the form of a current. Output values of a mass spectrometer to be used are calibrated with the use of a standard gas before the measurement of the helium concentration.