This invention provides an image processing method for projecting a virtual ray onto volume data to create an image and making it possible to realize speeding up without performing preprocessing. A virtual ray 22 is projected from an eye O onto an object 21, a position P at which the remaining light amount of the virtual ray 22 first attenuates is acquired, and a traveling distance d of the virtual ray 22 at the position P is acquired. To project an adjacent virtual ray 23, the virtual ray 23 is projected from a position O2 offset distance Δd smaller than the traveling distance d from the eye O by a predetermined value. Thus, when projection processing is performed, depth information d of the position P at which a first object is acquired on the virtual ray is stored and an adjacent virtual ray is projected from the position O2 offset Δd slightly smaller than the depth d, thereby skipping calculation.