PURPOSE: To reduce the power consumption of a mass spectrometer and miniaturize the spectrometer and reduce the cost of the spectrometer by setting the degree of vacuum of a first actuation area to less than 1Torr.

CONSTITUTION: Samples are introduced into atmospheric pressure plasma 10 generated in the area (I) of atmospheric pressure on a plasma generating system so that the samples are ionized. The sample ions are expanded and diffused in the area (II) of 1 to 10-3Torr via a plasma sampling system 70 and an ion beam composed of the sample ions, etc., is formed by an ion takeout system provided in the area (III) of 10-4 to 10-5Torr and consisting of an ion takeout electrode 80 and an ion accelerating electrode 90, etc. The ion beam is transported via an ion transporting system 100 to a mass spectrometry system 110 provided in the area (IV) of 10-6 to 10-7Torr and the samples are identified by an ion detecting system 120.