It is well known that competing interactions such as electrostatic repulsive and van der Waals attractive forces dominate the dynamics of pattern formation in a variety of structures found in phase-separated Langmuir monolayers. However, the microscopic details of these forces are still largely left unresolved because of the technical limitation on measurements. The scanning Maxwell stress microscope, which we have been developing as a new tool for microscopic observation of electrostatic interactions in organic and biological systems, is a type of scanning probe microscope, designed to image the distribution of the Maxwell stress field over the sample surface. With this technique we have succeeded in obtaining images of the surface potential of a phase-separated Langmuir-Blodgett monolayer of phospholipid with submicron resolution.