A laser diode interferometer that uses an optical fiber is proposed. The laser diode simultaneously functions as a light source, a phase modulator, and a phase compensator. Detection of the vibration and reduction of external disturbance are carried out using a feedback control for the injection current of the laser diode. Thus it can measure vibration accurately in real time with a simple signal processing circuit. The optical fiber not only functions as a flexible probe, enabling us to measure the vibration of an object that cannot be moved onto the optical bench, it also acts as a reference mirror, thereby simplifying the unit construction. The principle of the feedback control in laser diode interferometer is also described in detail. © 1997 Society of Photo-Optical Instrumentation Engineers.