An analyzer carrying out analysis with a high degree of qualitative accuracy by separating impurities which hinder the analysis while analyzing in real time. An introducing tube is branched with one branched tube directly leading to an ion source for real time analysis. The other branched tube leads to the ion source so as to introduce the sample gas into the ion source at a later time. When a result of the real time analysis shows that a spectrum pattern on a MS spectrum is changed and an increase in the concentration of an impurity is observed, the introducing path for the sample gas is changed so that the sample gas is introduced into the ion source after the impurity is separated at the separation part.