1. An image processing device for generating a display image on the basis of three-dimensional data, comprising: a reader for reading and storing three-dimensional data indicative of a structure of a virtual three-dimensional object constructed by a set of specific structures oriented in a predetermined direction into a predetermined storage; a projection plane setter for setting a directional relationship between a virtual plane of projection on which said virtual three-dimensional object is projected and said virtual three-dimensional object; a reference position setter for setting a reference position of a specific structure included in said set of specific structures; a basic image generator for generating a basic image by projecting said specific structure oriented in said predetermined direction onto said virtual plane of projection in accordance with said directional relationship and, also, recognizing a reference image position corresponding to said reference position in said basic image; a detector for detecting each of reference projection positions in which said reference position of each specific structure included in said set of specific structures is projected to said virtual plane of projection at a time of projecting said virtual three-dimensional object onto said virtual plane of projection; an adjusted image generator for generating adjusted basic images on the basis of said basic image by adjusting a positional relationship between said basic image and said virtual plane of projection in accordance with said each of reference projection positions detected by said detector; and a display image generator for generating a display image by combining said adjusted basic images to each other wherein said basic image generator detects a reference projection position corresponding to said reference position of said specific structure and a reference projection distance as a distance between said reference position and said reference projection position, detects each of projection distances as distances between predetermined points on said virtual plane of projection and said specific structure, and gives a set of a projection distance value of said each of projection distances using said reference projection distance as a reference and a pixel value according to said each of projection distances to each of pixels of said basic image respectively, thereby generating said basic image, said detector also detects each of reference projection distances corresponding to said each of reference projection positions, said adjusted image generator adjusts each said positional relationship, each said projection distance value and each said pixel value in accordance with said each of reference projection positions and said each of reference projection distances detected by said detector on the basis of said basic image as a base, thereby generating said adjusted basic images, and said display image generator employs a set of a projection distance value indicative of a minimum projection distance and a pixel value corresponding to said minimum projection distance for a projection position in which a plurality of said adjusted basic images overlap in said virtual plane of projection at a time of combining said adjusted basic images to each other.