Japanese Departnebt of Information Physics and Computing, Graduate Scool of Information Science and Technology, the University of Tokyo, Japan, Sagayama/Ono Laboratory Master Degree Student 7-3-1, Hongo, Bunkyo-ku,¢©113-8656 Tel & Fax: 03-5841-6902 E-mail: nishiki[at]hil.t.u-tokyo.ac.jp