1. Kamioka Observatory, ICRR, The University of Tokyo, Hida, Gifu 506-1205, Japan
2. Research Center for Cosmic Neutrinos, ICRR, The University of Tokyo, Kashiwa, Chiba 277-8582, Japan
3. Institute for Cosmic Ray Research, The University of Tokyo, Kashiwa, Chiba 277-8582, Japan
4. Warsaw University of Technology, Institute of Radioelectronics and Multimedia Technology, Nowowiejska 15/15, Warsaw, Poland
5. National Center for Nuclear Research, Andrzeja Soltana 7, 05-400 Otwock, Swierk, Poland