A method and apparatus for measuring the concentration of a trace nitrogen dioxide contained in the atmosphere on a pptv level with excellent accuracy by a single-wavelength laser induced fluorescence method. The device comprises a laser light source unit, a light excitation unit, a light guide unit, an air passing unit for passing either the air to be measured or a reference air prepared by removing nitrogen dioxide from the air to be measured, a light sensor unit and a control unit. A single-wavelength laser beam is applied to the air to be measured and the reference air. The intensity of the light emitted from the air to be measured and the intensity of the light emitted from the reference air are measured, and the concentration of nitrogen dioxide is determined by using the difference between the two.