A calibration method for calibrating a gas concentration measuring apparatus including a gas concentration measuring space, the calibration method comprising the steps of: performing a variation value calculation procedure under each of a plurality of types of temperature conditions (Te) in which a reference gas contained in the concentration measuring space has different temperatures, the variation value calculation procedure comprising, measuring a propagation time (t) required for an ultrasonic wave to travel through a measurement track within the concentration measuring space; acquiring a temperature calculation value (Tcal) based on a measurement value of the propagation time and a reference length of the measurement track; measuring a temperature in the concentration measuring space to acquire a temperature measurement value (Tm); and acquiring a substituted temperature variation value (δ) indicative of a difference between the temperature calculation value (Tcal) and the temperature measurement value (Tm); and determining calibration information used for associating a temperature of a gas to be measured with a temperature compensation value (Δ) for the temperature, based on the substituted temperature variation value (δ) acquired under each of the temperature conditions wherein the reference length is acquired by setting the gas concentration measuring apparatus in a reference state where the reference gas is contained in the concentration measuring space whose internal temperature is set at a reference temperature; measuring a reference propagation time required for the ultrasonic wave to travel through the measurement track when the gas concentration measuring apparatus is in the reference state; and acquiring, as the reference length, a length of the measurement track based on the reference propagation time. performing a variation value calculation procedure under each of a plurality of types of temperature conditions (Te) in which a reference gas contained in the concentration measuring space has different temperatures, the variation value calculation procedure comprising, measuring a propagation time (t) required for an ultrasonic wave to travel through a measurement track within the concentration measuring space; acquiring a temperature calculation value (Tcal) based on a measurement value of the propagation time and a reference length of the measurement track; measuring a temperature in the concentration measuring space to acquire a temperature measurement value (Tm); and acquiring a substituted temperature variation value (δ) indicative of a difference between the temperature calculation value (Tcal) and the temperature measurement value (Tm); and measuring a propagation time (t) required for an ultrasonic wave to travel through a measurement track within the concentration measuring space; acquiring a temperature calculation value (Tcal) based on a measurement value of the propagation time and a reference length of the measurement track; measuring a temperature in the concentration measuring space to acquire a temperature measurement value (Tm); and acquiring a substituted temperature variation value (δ) indicative of a difference between the temperature calculation value (Tcal) and the temperature measurement value (Tm); and determining calibration information used for associating a temperature of a gas to be measured with a temperature compensation value (Δ) for the temperature, based on the substituted temperature variation value (δ) acquired under each of the temperature conditions wherein the reference length is acquired by setting the gas concentration measuring apparatus in a reference state where the reference gas is contained in the concentration measuring space whose internal temperature is set at a reference temperature; measuring a reference propagation time required for the ultrasonic wave to travel through the measurement track when the gas concentration measuring apparatus is in the reference state; and acquiring, as the reference length, a length of the measurement track based on the reference propagation time.