In order to develop the nondestructive outgoing inspection method that determines the surface conditions in the induction-hardened carbon steel, X-ray diffraction method was applied both to estimate the surface hardness and to predict the hardened zone depth using micro X-ray beam adjusted by the pinhole collimator. In this study, S48C hardened specimen cut from the root circle of drive gear was used. The Vickers hardness distribution and 2/5-value breadth of X-ray diffraction patterns of 100,200 and 211 planes for ferrite and martensite phases were measured from the hardened zone, and then a correlation between hardness and 2/5-vale breadth was examined. As a result, the hardness of the induction-hardened zone was proportional to the 2/5-value breadth. From the obtained relationship, the surface hardness could be nondestructively estimated by measuring the 2/5-value breadth from the induction hardened carbon steel. Furthermore, by making a master curve for the subsurface hardness distribution of the mass produced parts in the optimum condition, the residual hardened zone depth could be also predicted from the measured 2/5-value breadth. The predicted hardened zone depths in the induction hardened drive gear agreed very well with the results measured by removing the surface layer.