1. An image converter for performing processes of cutting out a part of a distorted circular image photographed by use of a fisheye lens and converting said part into a planar regular image, the image converter comprising: a distorted circular image memory for storing a distorted circular image constituted with an aggregate of many pixels arranged at a position indicated by coordinates (x, y) on a two-dimensional XY orthogonal coordinate system and having a radius R taking an origin O of the two-dimensional XY orthogonal coordinate system as a center; a planar regular image memory for storing a planar regular image constituted with an aggregate of many pixels arranged at a position indicated by coordinates (u, v) on a two-dimensional UV orthogonal coordinate system; a parameter input unit in which in a three-dimensional XYZ orthogonal coordinate system including the two-dimensional XY orthogonal coordinate system, a visual line vector n facing any given direction, with the origin O given as a starting point, is input as a parameter indicating a cut-out position of the planar regular image, a predetermined planar inclination angle φ is input as a parameter indicating a cut-out orientation of the planar regular image, and a predetermined magnification m is input as a parameter indicating a cut-out dimension of the planar regular image; a corresponding coordinate calculating unit calculating corresponding coordinates (x, y) which correspond to any given coordinates (u, v) by using predetermined correspondence relationship equations showing a correspondence relationship between coordinates (u, v) on a two-dimensional UV curved coordinate system and coordinates (x, y) on the two-dimensional XY orthogonal coordinate system, wherein said two-dimensional UV curved coordinate system is defined by curving a two-dimensional UV orthogonal coordinate system which is arranged on a plane passing through a point G given as an origin and orthogonal to the visual line vector n to have an orientation according to the planar inclination angle φ, said point G being away from the origin O by “a product m·R of the magnification m and the radius R” on the visual line vector n, along a side face of a “virtual cylindrical column in which the point G gives one point on the side face thereof to have a central axis parallel to a V axis of the two-dimensional UV orthogonal coordinate system,”; and a planar regular image forming unit giving coordinates (u, v) of a target pixel constituting the planar regular image to the corresponding coordinate calculating unit to obtain corresponding coordinates (x, y), reading out a pixel value of a pixel arranged in the vicinity of the obtained corresponding coordinates (x, y) inside the distorted circular image memory, determining a pixel value of the target pixel on the basis of a read pixel value, thereby forming the planar regular image by determining pixel values of individual pixels, and writing the pixel values into the planar regular image memory.