An image processing apparatus for generating a projection image on a projection plane, comprising: an image obtaining unit configured for obtaining a three-dimensional medial image, in which the position of each pixel is defined by a three-dimensional coordinate system and the pixel value of each pixel being related to the coordinates of the position of the pixel, a display setting unit configured for setting an intended display area and a non-display area in the three-dimensional image, and setting a mask having display/non-display information with respect to each pixel of the three-dimensional medical image a search point setting unit configured for setting a plurality of search points (Pji) in the three-dimensional image, by sampling the intended display area of the three-dimensional medical image at a predetermined interval along a plurality of visual lines (Ej) connecting a viewpoint and each pixel on the projection plane a pixel calculation unit configured for determining whether or not each of eight pixels adjacent to each search point (Pji) is a pixel belonging to the non-display area with reference to the mask having display/non-display information, and the pixel calculation unit further configured for calculating a pixel value g (Pji) at each search point (Pji) by performing a linear interpolation on eight adjacent pixels forming a grid in the three-dimensional image that includes the search point (Pji), and a projection value determining unit configured for determining a pixel value of each pixel on the projection plane with respect to each visual line based on the pixel values of the search points (Pji), and generating a projection image formed of each pixel (Pji) on the projection plane, characterized in that if an adjacent pixel of a search point (Pji) is a pixel in the non-display area, the pixel calculation unit is adapted to replace the pixel value of the adjacent pixel in the non-display area with a value calculated from a pixel value of a pixel in the intended display area, by replacing the pixel value of the adjacent pixel in the non-display area with a maximum value of all pixel values of pixels in the intended display area or, by replacing the pixel value of the adjacent pixel with a maximum, or average value, of pixel values of other adjacent pixels belonging to the intended display area. an image obtaining unit configured for obtaining a three-dimensional medial image, in which the position of each pixel is defined by a three-dimensional coordinate system and the pixel value of each pixel being related to the coordinates of the position of the pixel, a display setting unit configured for setting an intended display area and a non-display area in the three-dimensional image, and setting a mask having display/non-display information with respect to each pixel of the three-dimensional medical image a search point setting unit configured for setting a plurality of search points (Pji) in the three-dimensional image, by sampling the intended display area of the three-dimensional medical image at a predetermined interval along a plurality of visual lines (Ej) connecting a viewpoint and each pixel on the projection plane a pixel calculation unit configured for determining whether or not each of eight pixels adjacent to each search point (Pji) is a pixel belonging to the non-display area with reference to the mask having display/non-display information, and the pixel calculation unit further configured for calculating a pixel value g (Pji) at each search point (Pji) by performing a linear interpolation on eight adjacent pixels forming a grid in the three-dimensional image that includes the search point (Pji), and a projection value determining unit configured for determining a pixel value of each pixel on the projection plane with respect to each visual line based on the pixel values of the search points (Pji), and generating a projection image formed of each pixel (Pji) on the projection plane, characterized in that if an adjacent pixel of a search point (Pji) is a pixel in the non-display area, the pixel calculation unit is adapted to replace the pixel value of the adjacent pixel in the non-display area with a value calculated from a pixel value of a pixel in the intended display area, by replacing the pixel value of the adjacent pixel in the non-display area with a maximum value of all pixel values of pixels in the intended display area or, by replacing the pixel value of the adjacent pixel with a maximum, or average value, of pixel values of other adjacent pixels belonging to the intended display area.