<p>The method for analyzing an impurity in a gas comprises the steps of: introducing a gas with an impurity into a first cell (2); introducing a gas with no impurity into a second cell (3); maintaining identical pressures in the first and second cells; irradiating a light from a light irradiating means (1); splitting the light by a splitting means (4) in order to pass a first beam trough the first cell and to pass a second beam through the second cell; measuring the intensity of the light passing through the first cell by a first measuring means (5) and the intensity of the light passing through the second cell by a second measuring means (6); and determining an absorption spectrum of the impurity in the gas based on the difference between measuring data from the first measuring means and measuring data from the second measuring means.</p>
