On May 17 last year, lawmaker Tomohiro Ishikawa, arrested for allegedly falsifying a political funds report but free on bail, walked into the Tokyo District Public Prosecutor's Office with a mission — to record his pending interrogation.

With a small IC recorder hidden in his bag, Ishikawa spent five hours being grilled by the prosecutor.

The recording, later disclosed and used as evidence in court, showed the prosecutor pressured Ishikawa by warning he may be handed a fresh arrest warrant if he changed his initial claim.