On May 25, 1910 (in what has become known as the Meika Incident), Miyashita was arrested in Nagano Prefecture on suspicion of violating the Explosives Control Act, and on suspicion that Miyashita and his associates, including Suga KANNO, Tadao NIIMURA, Rikisaku FURUKAWA, were planning to assassinate the Meiji Emperor; this incident subsequently triggered the High Treason Incident, due to the suspicion that some of them had plotted to assassinate the Emperor under the leadership of Shusui KOTOKU.
1910年5月25日、爆発物取締罰則違反の容疑により長野県で逮捕され（明科事件）、宮下は菅野スガ、新村忠雄、古河力作らと共に天皇暗殺の構想を抱いていたとされ、数人は幸徳秋水を首領に天皇暗殺を企てたとして大逆事件の端緒を開いた。