Geophysical prospecting using electrical resistivity is one of the principal methods for subsurface exploration. However, the majority of such application methods are restricted to coarse descriptions of underground conditions. The Q-system is commonly used as a representative rock mass classification system in modern rock engineering. In this paper, electrical resistivity is related to the Q-system through theoretical analyses. The analyses are based on Coulomb's law and Gauss' law considering electrical characteristics of constituent parameters for rock mass classification such as joint thickness, joint condition, joint spacing, intact rock strength, and RQD. The results show that there is a strong correlation between electrical resistivity and rock mass classification.