A former Air Self-Defense Force colonel pleaded not guilty to an alleged breach of confidentiality in the first hearing of his trial at the Tokyo District Court on Wednesday.

According to his indictment, So Kanno, the 63-year-old defendant, is alleged to have leaked information in 2013 about the systems of E-2D early warning planes provided by the U.S. government to two employees of a domestic defense trading company, in violation of the secret protection law incidental to the Japan-U.S. mutual defense assistance agreement.

In an opening statement, prosecutors claimed that Kanno had received a compact disc and documents containing information described as confidential when he had a briefing about the E-2D aircraft's structure and capabilities from a U.S. military official and others in September 2010. They said the possible leaking of the information may cause severe problems for security.

The defense said that the U.S military official told Kanno that the data was not actually secret and that the official passed it to him personally. The defendant subsequently gave an explanation about the E-2D to the two employees, but this was not regarded as secret and Kanno never showed or passed confidential documents, the defense stated.

Kanno allegedly passed a USB flash drive containing data on special defense secrets about the E-2D to the two workers, while showing them the data on a computer display on Jan. 9, 2013, at the ASDF's Iruma base in Saitama Prefecture, according to the indictment.