An information system can store personal information of its primary users such as shopping histories, and some third party wants or happens to know such information. Because the system usually provides its privacy policy and its users have to give their consent to it, they sometimes have to partially give up the protection of their privacy. On the other hand, a chance of a third party to know such information is too limited if the policy is too defensive. We proposed a method to explore trade-offs between protection of such information and access permissions for a third party, and exemplified it. In this method, operation logs of a system are focused. The structure of each log is then modelled for analysing what kinds of information can be accessed by a third party. Access limitations of each third party are explored so as to balance the protection of privacy information against access right of third parties.