The performance of the CT scanner "TCT-700S" (Toshiba Medical), including noise, spatial and contrast resolution, and exposure, was evaluated. Noise increased as the slice thickness decreased. No matter what the slice thickness, noise was great when the product of tube current and scan speed (described by mAs in the following sentences) was small. Furthermore, the short scan speed caused considerable large noise. Reduction of the mAs value caused remarkable increase in noise when the slice thickness was 1 mm or 2 mm. At combinations of slice thicknesses of 1 mm or 2 mm and 100 mAs, noise was 1-1.5%. When slice thicknesses were measured as the spatial resolution, in the case of a slice thickness of 1 mm, the measurements were 1.6 to 1.8 mm. When high and low contrast resolutions were measured, maximum diameters for the recognition were large as slice thickness became thinner. Although average exposure promised the improvement in the contrast resolution, great exposure increases failed to contribute to such improvement. These performances depended on the exposure. In the case of multiple scanning, average exposures in the field were uniform. However, at a minimum scan speed of 1.8 sec, a three fold discrepancy occurred in exposures observed reciprocally from the left and right sides. Average exposures were also almost the same with slice thicknesses of 10 mm, 5 mm and 2 mm, but with a slice thickness of 1 mm, average exposure was twice as great. This measurement indicates inadequate collimation in the case of a slice thickness of 1 mm. These average exposures were proportional to the mAs value. The radial-exposure profile depended on the position in the columnar object. Although similar average exposures were observed, high peak values were observed on the upper side, but exposure tended to spread at the bottom in the center. This means that the primary x-ray amounts were great in the peripheral region and scattered x-ray amounts were great at the center.