Abstract : As the networks become large-scale, they include various confidential data which need strict access control. Then we face the problem of satisfying the conflicting requirements of confidentiality and utilization. Access should be allowed when the information is necessary in performing the duty, although, in some cases, full access is not necessary when the data is used to compute statistics. Such unnecessary disclosure is required under existing access-control system. A typical case is the medical information system. For example, it should be kept confidential that "Mr. Smith of age 37 is suffering from hypertension." In some public health studies, scientists might have to compute "mean blood pressure of men in their thirties." In such cases, they do not have to know the blood pressure of Mr. Smith, but in the existing access-control system, it must be disclosed just in order to compute the average. Data processing, such as computing average and correlation, is enabled without disclosing the actual value, by using cryptosystem and cryptographic protocol. We propose a medical network system utilizing the new access-control system and new cryptographic protocol, thereby realizing active data sharing and interoperability between medical network and non-medical networks, such as disaster countermeasures or nursing care network.