1. An image processing apparatus, comprising: mapping image generating means, for generating a mapping image M that represents a portion of a subject, by employing a first voxel data set, which is a voxel data set of a three dimensional image that represents the subject with a degree of opacity assigned to each voxel thereof; aspect image generating means, for generating an aspect image S that represents the aspect of a target which includes at least a portion of the subject from a second voxel data set of the three dimensional image that represents the subject with a degree of opacity assigned to each voxel thereof; and image generating means, for generating a pseudo three dimensional image, by matching the mapping image M and the aspect image S and executing volume rendering, wherein the image generating means executes volume rendering by: setting an alpha value based on a degree of opacity α m of each determined voxel v i ′ of the mapping image, for performing alpha blending processing, wherein said determined voxels v i ′ of the mapping image are determined as voxels v i ′ of the mapping image which correspond to voxels v i of the aspect image which are to be used in volume rendering, and obtaining a pseudo three dimensional image having pixel values C by blending the mapping image and the aspect image with formula (4): C = ∑ i = 1 n ⁢ ( ( 1 - α m ⁡ ( v i ′ ) ) ⁢ c s ⁡ ( v i ) + α m ⁡ ( v i ′ ) ⁢ c m ⁡ ( v i ′ ) ) × α s ⁡ ( v i ) ⁢ ∏ j = 1 i - 1 ⁢ ⁢ ( 1 - α s ⁡ ( v j ) ) ( 4 ) wherein c s are brightness values, and α s are degree of opacity for each point v i of the aspect image to be used in volume rendering, and c m are brightness values, and α m are degree of opacity for each determined voxel v i ′ of the mapping image, wherein said obtaining comprises: calculating, with respect to said each point to be used for the volume rendering, a blended brightness value with formula (4), by performing alpha blending processing for each of said determined voxel v i ′ of the mapping image, to obtain a calculated blended value, by applying said alpha values to: the brightness values c m for each point v i ′ of the mapping image to be used in volume rendering, and the brightness values c s of the points v i in the aspect image to be used for the volume rendering, and executing volume rendering with formula (4) based on the calculated blended value and the degree of opacity α s assigned to each voxel v i of the aspect image used in volume rendering.