We analyzed volume changes in the computerized tomography(CT) and the magnetic resonance imaging(MRI) utilizations. These two volumes are comparative because CT has been covered by the Medical Insurance from the beginning of 1996, however, MRI has not been covered up to now. We investigated 80 hospitals which afforded a facility of CT by the end of 1994. For the comparison of CT utilization, we selected two periods. The first period was from January to June of 1995, in which CT had not been covered by the Medical Insurance and the second period was from January to June of 1966, in which CT had been covered. The information we obtained were characteristics of hospitals, patients numbers per month, monthly uses of CT and MRI, and payments of CT claims, etc. After the Medical Insurance coverage, the number of CT uses per 1,000 patients were increased from January to March, however, was decreased from April to June in 1996 compared to the same periods of 1995. the number of CT uses in clinics were large than those in tertiary hospitals. The number of CT uses in small cities were larger than those in big cities. On the other hand, the number of MRI uses per 1,000 patients did not changed from January to May and was increased from June in 1996 compared to the same periods of 1995. These results showed the substitutional relationship of CT and MRI uses. After the insurance coverage for CT, the utilization of CT was increased because patients payed less than before. However, insurers restricted the payments to the CT claims. This impact might explain the substitutional relationship between CT and MRI.