The spectroscopic film thickness measurement system is generally composed of a microscope, a spectrometer, and a data processing unit, and it enables non-contact measurement of transparent films (such as SiO2) on semiconductor substrate with a minute spot objective of some micron meter in diameter. Its measurement principle is based on the interference effect of multiply reflected lights from film and substrate surfaces, and the measurement result is obtained by analyzing the spectral profile of the reflected lights separated by the spectrometer. The measurement accuracy depends on many factors. The accuracy has been already often discussed, however, few reports have referred in detail to the relation between the measurement accuracy and NA characteristic of objective. This report will deal with this NA-relating issue. This paper explains: (1) the factors that have an influence on the accuracy of the spectroscopic film thickness measurement, (2) influence by the NA characteristic of objective, (3) our spectroscopic film thickness measurement system, and (4) summary.