A mass spectrometer for analyzing trace impurities on a level between ppt and ppb contained in silicon material gas such as monosilane gas. The mass spectrometer includes an ion formation region, reaction region, and mass analysis region. Ion formation gas is introduced into the ion formation region and sample gas (silicon material gas) is introduced into the reaction region. The ion formation region ionizes ion formation gas by an ionizer and forms primary ions. When the pressure of ion formation gas is made higher than the pressure of sample gas, the ion formation gas flows into the reaction region from the ion formation region together with primary ions and is mixed with the sample gas. In the reaction region, an ion-molecule reaction is produced between the primary ions and trace impurities contained in the sample gas and the trace impurities contained in the sample gas are ionized. The ion intensity of trace impunities, the concentration of trace impurities in the sample gas is determined using a calibration curve. When the gas pressure in the reaction region is kept at almost 1 atmosphere, the reaction is promoted and when the ion-molecule reaction time is optimized according to the size of the reaction region and the voltage condition, impurities on a level between ppt and ppb can be detected and determined.