An interferometric range finder that uses both phase-locked and modulation-amplitude-locked techniques in a sinusoidal phase-modulating interference signal is described. It allows us to perform unambiguous and real-time distance measurement. Also, external disturbance superimposed on the interference signal is eliminated by the feedback control. Our preliminary experiments indicate a measurement error of 17.7 mum rms. (C) 2004 Society of Photo-Optical Instrumentation Engineers.