The sensitivity factor of an axial symmetric transmission gauge (AT gauge) was determined by the conductance modulation method (CMM) in the pressure range from 10(-10) to 10(-6) Pa. The gauge has a Bessel-box type energy filter between a Bayard-Alpert gauge type ionizer and an ion detector to eliminate the limiting factors of pressure measurement in extreme high vacuum such as the soft x-ray effect and the electron stimulated desorption of ions. The lower limit of the pressure measurement of the gauge is estimated to be about 10(-12) Pa or lower. The sensitivity factor showed a slight pressure dependence caused mainly by outgassing from the gauge itself. The sensitivity factor of the AT gauge, which was further corrected for the effect of outgassing by the CMM, is almost constant in the pressure range from 10(-10) to 10(-6) Pa and is (2.3 +/- 0.1) x 10(3) Pa-1 for hydrogen.