As a method for the characterization of Langmuir Blodgett (LB) films, a scanning surface potential microscope was built by modifying a commercial atomic force microscope. The present microscope could be used to see the lateral distribution of work functions of a surface with patterns of two kinds of metals and used to give topographic images of patterned surfaces covered with one or two kinds of metals. For conductive surfaces covered with LB films, the present method afforded information of the lateral distribution of surface dipole moments in a phase-separated monolayer as well as dielectric properties of monolayer. The principle of the present method will be discussed in the light of the measurement of the Volta potential differences. Not only phase separation but also orientation of amphiphiles in LB films will be investigated in a high lateral resolution.