There was previously no monitoring method and monitoring apparatus which could measure dioxins at ppt levels and dioxin precursors at ppb levels with high sensitivity. According to this invention, organic and inorganic compounds containing highly electronegative elements are selectively ionized by atmospheric pressure chemical ionization, the ions produced are detected by a mass spectrometer, and their amount is measured. As a result, interfering substances such as nitrogen, air, hydrocarbons and carbon dioxide which are the main components of flue gas are eliminated, and dioxins or organochlorine compounds such as dioxin precursors can be selectively monitored.