The phase analysis of fringes by applying moiré techniques to the interferometer is well-known. Since this method is based on 2-D subtraction method in the incoherent optics, the distortion of the optical system is easily removed and a large aperture interferometer can be constructed. In this paper, a novel automatic method to detect phase of fringes was proposed by combining the conception of moiré-interferometer with a microcomputer system. The validity of the method is shown by experimental results in the phase analysis of fringes of slide-glasses. Furthermore, this system was applied to the measurement of pressure distributions in a Laval nozzle, and the measured results approximately coincided with the theoretical values. These results suggest that the system is well applicable to the fields of fluid mechanics.