The scanning Maxwell stress microscope (SMM) is a versatile electric force microscope, which has been developed here to image a variety of electrical features of surfaces and thin films. By the SMM, imaging the surface potential distribution over phase-separated Langmuir-Blodgett monolayers of phospholipid deposited on metal substrates has been successful. For monolayers deposited at a surface pressure corresponding to liquid expanded-liquid condensed phase separation, the presence of circular domains with diameters around 1 mum has been clearly visualized, giving direct insight into the electrostatic interactions in the lipid monolayers.