Magnesium (Mg) has the lowest standard electrode potential in structural materials, and then it easily corroded for that property contacting with other metals. The corrosion resistance of Mg alloys was generally investigated by conducting saltwater immersion test, salt spray test, and electrochemical corrosion test. However, these tests give only macroscopic information but don’t give microscopic (i.e. galvanic corrosion at the interface). So, the origin of galvanic corrosion at the interface between dispersoids and a-Mg in Mg alloys was investigated for fundamental clarification. In this paper, the surface potential difference (VSPD) at the interface between dispersoids and a-Mg using AZ91D Mg alloy is measured by using Scanning Kelvin Probe Force Microscope (SKPFM). Surface potential of pure metals measured by SKPFM had good correlation with standard electrode potential (SEP), and then VSPD also positively correlated with difference of SEP. Salt water immersion test was conducted using AZ91D in order to compare VSPD with corrosion phenomenon. Topographic map around the interface between dispersoids and a-Mg was measured by using AFM after corrosion test. Corroded phase corresponded to the anodic phase indicated by surface potential.