To provide a gas analyzer capable of measuring the total pressure of gases of a vacuum atmosphere along with the partial pressure ratio thereof and accurately measuring the total pressure of the gases.

The gas analyzer consists of an ion source, consisting of a cylindrical grids and filaments arranged outside the grids to ionize gases, the lead-out electrode adjacent to one end part of the grid to be provided on the same axis as the center axis of the grid and extracting ions, a mass analyzer for discriminating the extracted ions, a partial pressure collector for detecting the ions discriminated by mass and the total pressure collector arranged on the center axis of the grid. Two regions which diffier in the direction electric field are formed in the grids at the same time or two states having different electric field directions are formed alternately, and the total pressure and partial pressure ratio of mixed gas can be measured by the current flowing to the total pressure collector and the partial pressure collector.