A VPN connection method and a communication system are provided, each which can omit the setting of a firewall and the installation of a relay server and can establish communications from the outside of a firewall to the inside thereof. The internal network 2 is connected to the external network 1 via the firewall 13. The internal network 2 enables communications by an e-mail from the external network 1 (outside the firewall) to the mail server 21 in the internal network 2 (inside the firewall) or communications by telephone or radio, which is not via the firewall 3. The feature of the firewall 3 is utilized that access from the outside to the inside of the firewall through VPN connection is very difficult but the access in the reverse stream can be simply performed. Using the mail server, telephone line or radio line, the internal network 2 receives an e-mail, which requires for VPN connection from the external network 1, or control information through the telephone line. Thus, the internal network 2 sends callback for VPN connection to the e-mail transmission source. As a result, the callback allows the external network 1 to pass through the firewall 3.