PURPOSE: To improve contrast resolution while suppressing the deterioration of space resolution as low as possible by executing filtering among picture elements corresponding to an objective slice and auxiliary slices positioned at least on both the sides of the objective slice.

CONSTITUTION: The 1st smoothing means 21 executes the smoothing processing of original slices G in respective objective slices, and when five original slices G e.g. are sampled, five smoothed slices H1 are obtained. The 2nd smoothing means 22 executes smoothing processing among an objective slice and two auxiliary slices arranged on both the sides of the objective slice e.g. and obtains one smoothed slice H2 from the five original slices G. Both the smoothing means 21, 22 can be optionally switched by a switching means 23 in accordance with a select signal. In addition, noise can be removed simultaneously with the smoothing processing. Consequently, contrast resolution can be improved while suppressing the deterioration of space resolution as low as possible.