In manufacturing electron tubes having an oxide coated cathode, exhaustion of gas given off by decomposition of coating material (BaSr) CO₃ is one of the processes that influence much the performance of the cathode. During the exhaustion process, we often encounter with insufficient decomposition of the carbonate, sintering of BaO crystals by over-heating and contamination due to oxidation of the core nickel and of the other parts in the tube, which are all undesirable. To study the influence of exhaustion process on the cathode performance, a servo pressure controller is used with which the evolution of gas by decomposition from the cathode is kept at a desired rate by controlling the cathode input automatically. Gas analysis showed that the quicker the decomposition as well as the exhaustion are carried out, the slighter is the contamination on the nickel surface even if the pressure of evolved gas is high. Contamination of core nickel seems to depend upon the duration during which the core nickel is exposed to evolved gas rather than the gas pressure in the tube.