This paper presents a new method of measuring high reflectivity-films in respect of their absolute reflectivity by using a spherical cavity optical delay line. This method is characterized by (1) Arbitrary adjustment of the number of reflections of a light beam on a sample mirror and the pattern traced by the light spot and hence a greater suitability to measurements of various kinds. (2) With reflectivity given in terms of its absolute value. (3) Simple in structure, easy for use, suitable for a light beam on a sample mirror and the pattern traced by the light spot and henoe a greater suitability to measurements of various kinds. (2) With reflectivity given in terms of its absolute value. (3) Simple in structure, easy for use, suitable for a general application to ordinary laboratories. This paper gives the theoretical calculation formula for designing an optical delay line and a full consideration of this idea used as a reflectivity measuring instrument. The paper also points out that it is possible to measure the transmissivity of low scattering samples with this instrument. Experimental results show that this method can be used to measure the reflectivity of high-reflectivity-films with a precision of better than 2X10-4.