To provide an access control system for performing access control, etc., in a case of detecting irregular behavior in a network, an access control system includes a message-obtainment-processing unit configured to obtain from an email server an email message including information for identifying a suspect-terminal that has conducted irregular behavior, the email message being generated in a security system located inside or outside a private network constituted by use of a software defined network (SDN) to be received by the email server. The message-obtainment-processing unit causes an SDN controller in the private network to provide, to the suspect-terminal or an edge network device that controls communication of the suspect-terminal, a control instruction based on the obtained email message, so as to cause the suspect-terminal or the edge network device to execute a process for access control against the suspect-terminal.