A gas monitoring apparatus capable of real-time detection of a kind of chemical warfare agent, namely diphenylcyanoarsine (DC) and/or diphenylchloroarsine (DA). Atmospheric pressure chemical ionization mass spectrometry is carried out in the positive ionization mode, the total amount of DC and DA is determined from the intensity of an ion common to DC and DA, the DC concentration is determined from the intensity of an ion specific to DC, and the difference between them is regarded as the DA concentration.