1. Master of Engineering Student, Dept. of System Design Engineering, Keio Univ., Hiyoshi 3-14-1, Kohoku-ku, Yokohama 223-8522, Japan.
2. Professor, Dept. of System Design Engineering, Keio Univ., Hiyoshi 3-14-1, Kouhoku-ku, Yokohama 223-8522, Japan (corresponding author).