1. Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
2. IN2P3-CNRS, Ecole Polytechnique, Palaiseau, France
3. Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia
4. Department of Physics, Kyoto University, Kyoto 606-8502, Japan
5. Department of Physics, Osaka City University, Osaka 558-8585, Japan
6. Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo, Hida, Gifu 506-1205, Japan