1. A coordinate computation device that transforms three-dimensional coordinates on a projection sphere to two-dimensional coordinates on a distorted image, comprising: a composite index computation unit for computing a composite index which is a combination of a height of a point of interest on the projection sphere and a distance of the point of interest from an optical axis which are obtained from said three-dimensional coordinates; a distance computation unit for computing a distance from an origin in said distorted image, of a point in said distorted image corresponding to said point of interest, from said composite index; and a coordinate computation unit for computing the two-dimensional coordinates in said distorted image, from the distance from said origin in said distorted image, wherein, when said three-dimensional coordinates are represented by (x, y, z), and a radius of said projection sphere is denoted by Rb, said composite index computation unit determines said composite index Rn according to: 
 Rn= √( x 2 +y 2 ) if z≦√ ( x 2 +y 2 ), and 
 Rn=√ 2× Rb−z if z<√ ( x 2 +y 2 ).