In this paper, we describe the measurement accuracy of numerical method detecting the depth of small tracer particles distributed in 3-D space. The detection method is based on the numerical reconstruction of in-line hologram patterns observed with a single CCD camera. In the computation of light intensity distribution, the numerical integration is performed for the domain up to the second dark fringe in order that the measurement accuracy is not affected by the depth of tracer particle. An averaging technique is employed for the radial light intensity profile to suppress the influence on the measurement accuracy of the difference between the origin of cylindrical coordinates in the numerical integration and the center of observed axisymmetric fringes. Performance test results show that the error of detected depth is not dependent on the difference between the origin of the coordinates and the center of the frinapes if the difference is less than 0.07(mm).