Acceleration below Thunder Clouds at Mount Norikura

New results have been obtained concerning particle acceleration beneath thunder clouds. In Summer 2001 an electric field mill was installed at Mount Norikura Cosmic Ray Observatory (2770 m). We have found that (1) the occurrence of particle events has a periodicity of 26 days, (2) there is evidence for proton acceleration in the presence of positive electric field, and (3) evidence has been obtained for increases of secondary cosmic rays under positive electric field.