We investigate the 'artifact' included in Kelvin probe force microscopy (KPFM). We theoretically show that the artifact due to the applied AC field cannot be canceled by conventional KPFM and that the artifact signal can be detected from the force signal through the second harmonic component of AC field. We made an experiment to simultaneously obtain the topography, the spatial distribution of the surface potential and the artifact signal on Si(1 1 1) surface using our noncontact atomic force microscope (NC-AFM). The result indicates that topography includes the artifact signal and that the surface potential image reflects the charge density of Si adatoms. We propose the novel method to completely eliminate the artifact due to the electrostatic force. (C) 2002 Elsevier Science B.V. All rights reserved.