An equidistant and straight fringe pattern, for example, Young's fringes, is a most fundamental and simplest fringe pattern in optics, but the measurements of the fringe profile, spacing, and its movement allow us many instrumentations. In optical sensings, various types of fiber sensors have been presented for detecting physical perturbations such as displacement, electric and magnetic fields, pressure, temperature, rotation velocity, and so on.l) Hocker used Young's fringes in a fiber-optic Mach-Zehnder interferometer and measured a fringe shift of an interferogram, which is proportional to a phase change of the light.2) However, a general method for detecting the fringe shift was not presented, which may provide highly sensitive optical sensings similar to that using a heterodyne detection. In this paper, a method for detecting a shift of Young's fringes is presented, which is based on an arctangent calculation with Fourier cosine and sine integrals of the fringes. In the phase detection, phase errors caused by a spatial truncation of the fringe, a nonlinearity of a detector, a sampling of the fringe, and a random intesity noise are analyzed theoretically. To demonstrate the present phase detection, experimental results for measurements of displacements of a PZT device are presented using non-polarized and polarized Michelson interferometers. As applications of the present phase detection, highly sensitive optical sensors for pressure, temperature, and displacement are proposed.