Objective: The aim of this study was to investigate the correlation between the demineralization depth measured by dental computed tomography (CT) and the measured value from a non-destructive dental caries diagnosis with a laser. Methods: Optimal cut-off points were obtained for enamel and dentin caries, from the measured levels from a dental caries diagnosis with a laser, to investigate the clinical usefulness of a diagnosis using a laser. Using human teeth, the demineralization depth was measured and the caries were diagnosed by a dental caries diagnosis with a laser. Results: The optimal cut-off point was investigated for accuracy, sensibility, and specificity, which were calculated from the results. These results demonstrated the correlation between the measured values and demineralization depth in both the pit and fissure caries and smooth surface caries. The optimal cut-off points were found to be 16–21 in the fissure caries and 9–11 in the smooth surface caries.