A gas analysis method includes irradiating a sample gas introduced into a gas cell with infrared light tuned to a wavelength corresponding to one absorption line of a target gas contained in the sample gas, measuring a sample signal value corresponding to intensity of transmitted light of the infrared light transmitted through the gas cell, evacuating the sample gas in the gas cell and then replacing by a reference gas, measuring a reference signal value corresponding to intensity of transmitted light of the infrared light transmitted through the reference gas, and calculating gas concentration at the one absorption line from ratio of the sample signal value to the reference signal value.