1. A method for calculating an intersection point of a triangle with a line segment, the triangle having three vertexes P 0 , P 1 , P 2 , and the line segment having both ends A and B, comprising: a coordinate system setting step (A) of setting a coordinate system R in which P 0 is a coordinate origin, P 0 P 1 conforms to a first axis (U axis) to have a unit length, P 0 P 2 conforms to a second axis (V axis) to have a unit length, and P 0 P 1 ×P 0 P 2 is a unit vector conforming to a third axis (N axis), a transforming matrix calculation step (B) of calculating a transforming matrix M that transforms coordinate values in an ordinary coordinate system into coordinate values in the coordinate system R; a line segment end coordinate calculation step (C) of calculating u-, v- and n-coordinate values of the both ends A and B of the line segment; an intersection determination step (D) of determining whether or not the line segment intersects with the triangle, on the basis of the u-, v- and n-coordinate values of the both ends A and B; an intersection point coordinate calculation step (E) of calculating u-, and v-coordinate values of the intersection point when the line segment intersects with the triangle; and an intersection point determination step (F) of determining whether or not the intersection point is positioned inside the triangle, on the basis of the u-, and v-coordinate values of the intersection point.