This paper describes an atomic force microscope (AFM) using an optical fiber heterodyne interferometer free from external disturbances in optical paths of two fibers used for measuring small displacement of a microcantilever. An array of two fibers is used for superimposing interference beams easily and obtaining two closely spaced spots on a microcantilever. For eliminating the disturbances, the phase of the two fibers and the two spots of measurement beat signal and the phase of the two fibers of reference beat signal are differentiated. The noise level of this interferometer is 1.1 X 10(-3) nm/square-root Hz and is limited by the phase noise of the phasemeter used. It is confirmed, by measuring thickness of a thin Au film, that the interferometer has a suitable performance for measuring the displacement of the microcantilever. By observing a surface of an optical disk, it is demonstrated that this AFM is a powerful tool for measuring a surface in the order of nanometer.