Long term behavioral characteristics of an axial symmetric transmission gauge, such as the outgassing rate and sensitivity factor, were examined over a period of 3500 h. It was found that the outgassing rate and the gain of the continuous-dynode-type electron multiplier, which was used for ion detection, were the main factors which determined the gauge characteristics. The outgassing rate of the multiplier as obtained from the manufacturer was very high but decreased to the acceptable level after about two days pf continuous operation in high and ultrahigh vacuum. The sensitivity factor of the newly fabricated gauge was very high but decreased with the decrement of the outgassing rate to the quasi-stationary state at which the factor tended to decrease gradually with operating time. When the gauge was exposed to atmospheric air, the sensitivity factor increased temporarily but decreased within the duration of the typical bakeout process. The increment was assumed to be due to the adsorption of gases on the multiplier surface. The lifetime of the electron multiplier estimated from the decay rate of the sensitivity factor during the acceleration test by hydrogen is about nine years for the operation of the gauge in the 10(-10) Pa region. It was also found that the gain of the multiplier was proportional to the soft x-ray background signal. The gradual variation in the sensitivity factor of the gauge caused by that of the gain of the multiplier over the period of operation can be compensated by this relation.