Disclosed herein is a method of measuring an absolute lung volume. In the method, concentrations of oxygen and carbon dioxide gases are measured and analyzed in respiratory gas, consisting of nitrogen (N.sub.2), oxygen (O.sub.2), and carbon dioxide (CO.sub.2), to indirectly measure a concentration of nitrogen, thereby achieving measurement of absolute lung volume, such as a functional residual capacity. The method comprises connecting a one-way valve to a subject so as to separate an inspiration path from an expiration path, measuring a flow rate of respiratory gas using a flow sensor, which is provided in the expiration path, continuously measuring a concentration (F.sub.O2) of oxygen and a concentration (F.sub.CO2) of carbon dioxide using O.sub.2 and CO.sub.2 sensors provided in the expiration path, correcting dynamic characteristics of the concentration (F.sub.O2) of oxygen and the concentration (F.sub.CO2) of carbon dioxide so that the dynamic characteristics agree with each other in terms of time, and analyzing oxygen and carbon dioxide gases using the following Equation.times..intg..times..times.d ##EQU00001##