The d.c. potential drop technique based on the use of adjacent probes is applied to the nondestructive evaluation of a crack bifurcated from a nonplanar crack in a rail. The method of evaluation is based on the measurement of the potential drop on the top surface of the rail. The problem of current flow in a material which contains nonplanar and bifurcated cracks is analyzed numerically using a 3-D finite element method. The relationship between the potential drop on the measured surface and the depth of a bifurcated crack is obtained and it is verified in experiments using rail specimens that the cracks can be characterized quantitatively using the 3-D numerical analysis. It is also shown that the sensitivity of the present method using adjacent probes is higher than that of the conventional method in which the uniform current flow in a region far from the crack is used.