First Author:   Yasunobu Tomoda Graduate School of Economics and Business Administration, Hokkaido University tomoda@pop.econ.hokudai.ac.jp   Kita-9, Nishi-7, Kita-ku Sapporo, 060-0809, Japan       Second Author: Hiroshi Kurata Graduate School of Economics and Business Administration, Hokkaido University kura@pop.econ.hokudai.ac.jp   Kita-9, Nishi-7, Kita-ku Sapporo, 060-0809, Japan