This study relates to mechanisms of automatic micro-analysis for gases in metals by the vacuum fusion method. Determination of extracted gases is automatically performed by recording the changes of pressure of gases gathered in a constant volume by a diffusion pump (heating current is constant) after eliminating each gas from others. We thought out a rotary McLeod gauge, which has stretched resistance platinum string in its capillary so as to enable us to measure electrically and to record automatically the pressures of gases as an electric current in the course of analysis. The rotary McLeod gauge is automatically rotated. The automatic selection of paths of gases, where the reactions of eliminating gases take place, are controlled with solenoid mercury cuts by time-switches on a rotary drum. The time necessary to determining O2 and N2 in a metal sample is less than 30 minutes including the time of extracting gases.