Scanning Maxwell stress microscopy (SMM), a type of scanning probe microscope capable of imaging the distribution of surface potential over the sample surface, was used to investigate the light induced electron transfer from cyanine dye to viologen moieties embedded in hetero-type Langmuir-Blodgett films. We could observe the light induced surface potential changes upon laser light illumination. Further, the direction of the changes was in accordance with that of the light induced electron transfer in hetero-type LB films.