A security wall, such as a firewall and a viruswall, is built easily which does not require firewall-dedicated hardware or viruswall-dedicated hardware nor, in a mobile information processing device, mobile terminal-dedicated hardware. For this purpose, on a single information processing device, a plurality of separate LAN segments are realized and data from an external network such as the Internet is forced to pass through the multiple LAN segments before it reaches a user system in order to reinforce the system against external attacks. The security wall system is made portable so that the firewall and the viruswall can be executed at the same time, strengthening the security of the mobile information processing device.