The Japanese Aviation Accident Investigation Board has two faces. While the surface of the Board is a one of the very accident investigation organ, its shadow face is a one of the very expert witness drawing up a requested written opinion for the sake of the criminal investigation of the Police under the two secret inter-ministerial accords. This Paper proposes that some evidence obtained by investigation ought to be protected for disclosure and use by privilege derived from on of our most basic legal principles: "Nemo debet se-ipsem accusare-no one is required to incriminate oneself-".