To provide a security system for establishing communication between an external network and an internal network while maintaining the reliability of a private line, in a security system for mediating the internal network and the external network.

The security system includes: an integrated management device connected to the external network; a management server connected to the internal network to monitor a device connected to the internal network; a monitoring device connected to the private line to perform monitoring for security; and a security control device connected to the external network, the internal network and the private line to receive information from the management server through the internal network, transmit the information to the integrated management device through the private line, receive information from the monitoring device through the private line, transmit the information to the integrated management device through the external network and transmit the information to the management server through the internal network.