The contact potential difference (CPD) imaging of Ag- and Au-deposited Si(111) 7×7 surfaces and the effect of CPD distribution on NC-AFM data are discussed. Scanning Kelvin probe force microscopy (SKPM) based on the measurement of electrostatic force gradient was applied under an ultrahigh vacuum (UHV) to acquire the data. The CPD images of Ag- and Au-deposited Si(111) 7×7 surfaces are virtually identical, irrespective of the deposited metals. Detailed analysis has revealed that the CPD images showing the atomically resolved potential difference, does not represent the intrinsic work function of the materials but reflects the local electron density on the surface. On the other hand, the average potential that corresponds to the DC level of the CPD image is considered to reflect the work function on the surface.