Abstract: An image synthesis system comprises a volume memory for accumulating entire volume data after being divided into cubic data comprising units each of which is composed of eight pieces of volume data for constructing a small cube to make it possible to perform memory operation by using the units of the cubic data; an image processing unit including a memory control unit, an inclination calculation unit, a brightness calculation unit, a synthesis calculation unit, and a coordinate conversion unit; an image display memory; and a display unit. The volume data is read from the volume memory by using the units each of which is composed of eight voxels for constructing the small cube. The inclination calculation is performed by the inclination calculation unit, the brightness is calculated by the brightness calculation unit, and the synthesis calculation unit is used to perform the synthesis calculation for the entire volume data along voxels through which an arbitrarily set imaginary line of sight passes.