Remaining life evaluation has been attracting much attention to secure the safety of aged power generating plants. A crack on the outer surface of pipe is a typical kind of damage observed in aged plants. Accurate information of crack largely helps the remaining life evaluation based on fracture mechanics. The direct-current potential difference method (DCPDM) is known as one of the suitable non-destructive techniques to monitor the initiation and growth of crack in the pipes. Usually, numerical analyses are necessary to evaluate the geometry of crack from the potential difference because no analytical solution was given in most cases. In this study, a solution of the potential difference around a semi-elliptical crack on the outer surface of pipe is presented and the result is compared with that obtained by finite element analysis.