Access control system

An access control method for performing access control in a private network constituted by use of an SDN, the access control method comprising:

• when a security system located inside or outside the private network detects irregular behavior in the private network, specifying a user terminal that has conducted the irregular behavior as a suspect-terminal and generating an email message including identification information of the suspect-terminal;
• storing the generated email message in an email server;
• causing an SDN controller or a predetermined computer, which controls communication in the private network, to obtain the email message stored in the email server;
• partially or entirely converting the obtained email message from a code that is not processable for the SDN controller into information that is processable for the SDN controller;
• providing, from the SDN controller to the suspect-terminal or an edge network device that controls communication of the suspect-terminal, a control instruction corresponding to the information converted into;
• and
• causing the suspect-terminal or the edge network device to execute a process for access control against the suspect-terminal, based on the control instruction.