A gas monitoring system for artificial ventilation includes: a sensor that is configured to produce a signal corresponding to a concentration of a predetermined gas in a portion which is in a respiratory circuit of an artificial ventilator, and through which both an inspiratory gas and an expiratory gas pass; a displaying apparatus that is communicable with the sensor; a processor; and a memory that is configured to store a command which is readable by the processor. When, during high-frequency oscillatory ventilation performed by the artificial ventilator, the command is executed by the processor, the processor is to configured to calculate a measurement value of the concentration based on the signal, and is configured to display at least one of a waveform corresponding to the signal and the measurement value on the displaying apparatus.