It is necessary for determination of crack size and shape by means of D.C. potential drop technique to know electric potential distribution at the surface of cracked body beforehand. The potential distribution on a body with semi-elliptical surface-crack is investigated in this paper. The results obtained are summarized as follows. (i) Exact distribution is derived as Eqs. (1)∼(3) for semi-infinite body with a semi-elliptical surface-crack. As it requires numerial integral to evaluate the potential, a simple approximation method (Eqs. (14) and (15)) is proposed for convenience. It is confirmed by numerical analysis that the error of approximation is very small. (ii) We propose an easy method to make a cracked body with an arbitrary crack shape and size for the calibration of electric potential distribution. Plate specimens with a semi-elliptical surface-crack are made of a wood metal, and the potential distribution is examined experimentally. The values measured coincide the solution of infinite body (see (i)) except that the crack depth is nearly equal to the thickness of plate. It implies that specimen's back surface affects fairly little on the potential distribution at the front surface.