PURPOSE: To prevent useless data transmission to a medium by detecting a fault not detected from an information processing terminal equipment of a transmission circuit and stopping power supply to the transmission circuit when the transmission circuit is faulty.

CONSTITUTION: A transmission circuit 5 sends data to a medium from an information processing terminal equipment. A reception circuit 6 sends received data to the information processing terminal equipment 1. A collision detection circuit 7 detects collision of data on the medium. Power used to drive the transmission circuit, the reception circuit and the collision detection circuit 7 is supplied through a power supply line 11. A transmission monitor section 8 compares a data transmission request from the information processing terminal equipment with a drive state of the transmission circuit and recognizes it to be a fault state when the transmission circuit is driven to send data to the medium regardless of no data transmission request and allows an external display section 10 to display it that the transmission circuit is faulty and a switch 9 is used to stop power supply to the transmission circuit.