1. An image processing apparatus comprising: a storage unit which stores a plurality of volume data; a first image generation unit to generate first superimposed images with respect to at least two of the plurality of volume data, each of the first superimposed images being a two-dimensional image obtained by superimposing information of one of at least two of the plurality of volume data along a first reference direction; a correspondence unit which performs spatial correspondence between the at least two volume data by using the first superimposed images; a second image generation unit to generate a second image by using the at least two volume data corresponded spatially, the second image having an image slice direction different from the first reference direction and corresponding to the same position or region between different volume data; and a display unit which displays the second image.