We describe a fringe locking phenomenon that is observed in a two beam interferometer using a laser diode. We found that the fringes were locked even when one mirror of the interferometer was vibrated at an amplitude of several wavelengths. The phenomenon is caused by optical feedback from the interferometer to the laser diode so that oscillation wavelength of the laser can be changed by mirror movement to compensate for the resultant change in the path difference. The phenomenon occurs if a fraction at least 0.1% of LD output power is returned. The degree of fringe locking depends on optical path difference periodically with a period of optical laser cavity length. The locking has also been observed for the mirror vibration at two wavelengths in amplitude and 10 kHz in frequency limited by the PZT driver. The wavelength shift caused by the mirror movement has been calculated using an equivalent Fabry-Perot cavity model for the laser diode interferometer and showed good agreement with experimental results.