Recently, X-ray CT (Computed Tomography) is very popular as helpful diagnostics. But its high exposure dose as compared with simple radiograph should be assessed on a clinical basis as an input to the justification and the optimization. The number of CT scanners in Japan, about 13,000, has been acknowledged to be the largest over the world. In Japan these situation urges the patient dose assessment on a clinical basis as an input to the justification and the optimization. Exposure dose in CT scans depend on scan conditions and patient’s age and body type. These conditions should be considered in dose evaluation in CT scans. In this study, we developed source models of various CT based on measurements, and calculated dose of tissues in CT scans using precise voxel phantoms and Monte Carlo code PHITS.