To obtain a high performance security system by specifying a device in which any illegal operation is executed or inhibiting any illegal operation by releasing a state that log-in is operated through a port to its own transmitting device or another transmitting device at the time of detecting cable pulling- out or the like at the port of one transmitting device.

The software program of a transmitting device (A station) 1 monitors the CTS (port cable pulling-out detection) and CD (inter-MODEM communication disconnection detection) bits of an SCCS state (Status) register corresponding to each port (numerals 1-5 with circle). Thus, when the cable pulling-out or inter-MODEM communication disconnection is detected, it is judged that log-in is being operated from which port to which device in a network by a log-in state management table. Then, all the log-in states (connection) through the port at which abnormality is detected is released.