An optical gas concentration measuring method using an optical gas concentration measurement means comprising a gas flow path, the optical gas concentration measurement means being arranged in a predetermined position in a gas supply line from which a raw material gas is introduced into a processing chamber in which a chemical process or a physical process is to be implemented upon said introduction, the method comprising the steps of: irradiating light having a first wavelength and light having a second wavelength onto the raw material gas in the gas flow path at separate times, wherein the raw material gas has a different light absorptivity for the light having the first wavelength and for the light having the second wavelength; receiving each of the light of the first wavelength and the light of the second wavelength that optically passes through the raw material gas according to the irradiating the light of the respective wavelength, using a common light-receiving sensor; integrating and storing a charge generated by the light entered into the light-receiving sensor in an ON state using an integrating amplifier or a storage capacitance provided in a latter stage of the light-receiving sensor; forming a differential signal between a first signal related to the light having the first wavelength and a second signal related to the light having the second wavelength; and deriving a concentration of a chemical component of the raw material gas on the basis of the differential signal, wherein each of the first signal and the second signal are formed by a difference between an integrated amount of the charge generated by the light having the respective wavelength received by the light-receiving sensor at the end of a respective first period having a width T13 and an integrated amount of the charge generated by the light having the respective wavelength received by the light-receiving sensor at the beginning of the respective first period, each of the respective first periods is arranged to be within a respective second period of width T12, during which the light-receiving sensor is in the ON-state for integrating the amount of the charge generated by the light having the respective one of the first wavelength and the second wavelength, and each of the second periods of the width T12 is arranged to be the same as or within a respective third period of width T11, during which the respective one of the light having the first wavelength and the light having the second wavelength is irradiated onto the raw material gas, so that the relationship is as follows: T 13 < T 12 ≤ T 11. irradiating light having a first wavelength and light having a second wavelength onto the raw material gas in the gas flow path at separate times, wherein the raw material gas has a different light absorptivity for the light having the first wavelength and for the light having the second wavelength; receiving each of the light of the first wavelength and the light of the second wavelength that optically passes through the raw material gas according to the irradiating the light of the respective wavelength, using a common light-receiving sensor; integrating and storing a charge generated by the light entered into the light-receiving sensor in an ON state using an integrating amplifier or a storage capacitance provided in a latter stage of the light-receiving sensor; forming a differential signal between a first signal related to the light having the first wavelength and a second signal related to the light having the second wavelength; and deriving a concentration of a chemical component of the raw material gas on the basis of the differential signal, wherein each of the first signal and the second signal are formed by a difference between an integrated amount of the charge generated by the light having the respective wavelength received by the light-receiving sensor at the end of a respective first period having a width T13 and an integrated amount of the charge generated by the light having the respective wavelength received by the light-receiving sensor at the beginning of the respective first period, each of the respective first periods is arranged to be within a respective second period of width T12, during which the light-receiving sensor is in the ON-state for integrating the amount of the charge generated by the light having the respective one of the first wavelength and the second wavelength, and each of the second periods of the width T12 is arranged to be the same as or within a respective third period of width T11, during which the respective one of the light having the first wavelength and the light having the second wavelength is irradiated onto the raw material gas, so that the relationship is as follows: T 13 < T 12 ≤ T 11.