Article 75 -That prime minister himself is a minister of state is made clear in Article 66. So if Ozawa prevails in the DPJ leadership contest against Kan Naoto and is elected Prime Minister, the Tokyo No. 5 Committee for the Inquest of Prosecution can indict him until they are blue in the face, but to no avail, unless of course Ozawa decides that for the good of the country he should be subject to the indictment.
The Ministers of State (naikaku kokumu daijin), during their tenure of office, shall not be subject to legal action without the consent of the Prime Minister. However, the right to take that action is not impaired hereby.
Which I am guessing he will not do.