To detect with high precision a relative minute motion of a point light source by making the light from a distant point light source interfered and then detecting changes in the interference pattern.

Light from a distant light source, such as a star, avoids stray light by a food means 10, and passes a filter means 11 for selecting a specified wavelength, and made incident on a slit means 12a having a slit of a specified interval. The light having passed each slit forms an interference fringe on a photo-detecting means 12 where optical sensor elements, positioned behind by as specified distance, and a detecting means 14 detects changes in the pattern of the interference fringe, for detecting relative minute motion of a point light source. If, instead of the means 12a, a phase difference board means using a transparent material is used, an interference fringe can be generated by using the light from a point light source more effectively, in addition, is a configuration wherein a lend system is assigned before the means 11 is employed. The light from a point light source can be made incident on the means 11 as a parallel light flux of a higher density.