This paper gives an analysis for the optical mechanism of the interferometric method of measuring the spectral slit width of the monochromator, suggested by V. J. Coates and H. Hausdorff. From the point of view of accuracy, moreover, it shows that the Michel-son interferometer is better than the Fabry-Perot interferometer.The spectral slit widths of type YM-2 monochromater for various slit widths were measured. The results are in agreement with the theoretical values.