Disclosed herein are an apparatus for and method of measuring the composition and the pressure of the discharged gas from an ion gauge by using a residual gas analyzer. In this regard, there are provided a vacuum container 200 divided into a pressure container 210 and a discharge container 220 by means of a partition 235 having an orifice 230 formed thereon; an ion gauge 100 mounted at the pressure container 210 side of the vacuum container 200 for discharging the gas at the time of vacuum formation; a residual gas analyzer 240 mounted at the pressure container 210 side of the vacuum container 200 for measuring the composition and the pressure of the residual gas; pump means disposed at one side of the discharge container 220 of the vacuum container 200 for discharging the inside gas; and heating means disposed at the vacuum container 200 for heating the vacuum container 200 to a predetermined temperature.