A gas monitoring apparatus includes a sample introducing portion, a
measurement portion, an ionization portion, a mass analysis portion, a
data processing portion and a display. The sample introducing portion
introduces a sample gas including an object material to be measured. The
measurement portion measures a concentration of a predetermined
coexisting material, which coexists with the object material in the
sample gas. The ionization portion ionizes the sample gas. The mass
analysis portion analyzes mass of an ion produced by the ionization
portion. The data processing portion analyzes signals detected by the
mass analysis portion to calculate a concentration of the object
material. And the display displays results of analysis conducted by the
data processing portion. The data processing portion includes an
adjustment portion which adjusts the concentration of the object material
according to the concentration of the predetermined coexisting material.