Compulsory investigation based on a court warrant

As indicated in recital 115, any data collection as part of a coercive investigation must be specifically authorised by law and may only be carried out based on a court warrant "issued for adequate cause" (Article 35 of the Constitution). As regards the investigation of criminal offences, this requirement is reflected in the provisions of the Code of Criminal Procedure ("CCP"). According to Article 197(1) of the CCP, compulsory measures "shall not be applied unless special provisions have been established in this Code". With respect to the collection of electronic information, the only relevant (82) legal bases in this regard are Article 218 of the CCP (search and seizure) and Article 222-2 of the CCP, according to which compulsory measures for the interception of electronic communications without the consent of either party shall be executed based upon other acts, namely the Act on Wiretapping for Criminal Investigation ("Wiretapping Act"). In both cases, the warrant requirement applies.
More specifically, pursuant to Article 218(1) of the CCP, a public prosecutor, a public prosecutor's assistant officer or a judicial police official may, if necessary for the investigation of an offence, conduct a search or seizure (including ordering records) upon a warrant issued by a judge in advance (83). Among others, such a warrant shall contain the name of the suspect or accused, the charged offence (84), the electromagnetic records to be seized and the "place or articles" to be inspected (Article 219(1) of the CCP).
As regards the interception of communications, Article 3 of the Wiretapping Act authorises such measures only under strict requirements. In particular, the public authorities have to obtain a prior court warrant that may only be issued for the investigation of specific serious crimes (listed in the Annex to the Act) (85) and when it is "extremely difficult to identify the criminal or clarify the situations/details of the perpetration by any other ways" (86). Under Article 5 of the Wiretapping Act, the warrant is issued for a limited period of time and additional conditions may be imposed by the judge. Moreover, the Wiretapping Act provides for a number of further guarantees, such as for instance the necessary attendance of witnesses (Articles 12, 20), the prohibition to wiretap the communications of certain privileged groups (e.g. doctors, lawyers) (Article 15), the obligation to terminate the wiretapping if it is no longer justified, even within the period of validity of the warrant (Article 18), or the general requirement to notify the individual concerned and allow access to the records within thirty days after the wiretapping has been terminated (Articles 23, 24).
For all compulsory measures based on a warrant, only such an examination "as is necessary to achieve its objective" – that is to say where the objectives pursued with the investigation cannot be achieved otherwise – may be conducted (Article 197(1) CCP). Although the criteria for determining necessity are not further specified in statutory law, the Supreme Court of Japan has ruled that the judge issuing a warrant should make an overall assessment taking into consideration in particular (i) the gravity of the offence and how it was committed; (ii) the value and importance of the materials to be seized as evidence; (iii) the probability (risk) that evidence may be concealed or destroyed; and (iv) the extent to which the seizure may cause prejudice to the individual concerned (87).