PURPOSE: To decide the optimum divided size of histogram of ρ and θ planes in the Hough transformation by deciding the optimum divided number of ρcorresponding to the θ in a certain divided size by introducing a mean integration square-low error standard (MISE standard), and considering any change in the optimum divided number of ρ corresponding to the change of θ.

CONSTITUTION: The MISE standard is introduced as the standard of optimization for the divided numbers of ρ and θ. Namely, concretely, the optimum divided number of ρ is decided so as to minimize an estimated error based on the cumulative MISE standards of ρ and θ planes corresponding to the θ in a sufficiently large divided size. Then the divided number of θ is reduce, the divided number of θ at the limitation to change the optimum divided number of ρ is decided as the optimum divided number of θ, and the optimum divided numbers of ρand θ are calculated in terms of MISE standard. Thus, the useless division of θ into fine parts is eliminated, and the optimum divided number of ρ and θcan be decided.