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.