The present invention relates to a method and an apparatus for measuring trace quantity of oxygen in a gas by reacting yellow phosphorus vapor and oxygen in a sample gas, and measuring the intensity of the light emitted by the reaction. Together with the sample gas, a constant amount of oxygen is continuously supplied to a reaction chamber so as to react with the yellow phosphorus vapor, and the intensity of the light emitted is measured. The oxygen concentration in the sample gas is determined from the difference between the value measured as above and the intensity of the light emitted by the reaction between the yellow phosphorus vapor and the added oxygen, or from the oxygen concentration obtained from the measured value and the concentration of the added oxygen. The constant amount of oxygen to be added is supplied by supplying a constant amount of a preliminarily prepared oxygen-containing gas together with the sample gas or by providing an oxygen permeating membrane in the yellow phosphorus vapor supplying system so as to supply the oxygen by the permeation of the oxygen in atmosphere. The small amount of oxygen with a concentration of several ppb in the sample gas can be continuously measured with high precision.