PURPOSE: To inhibit transmission to a network bus on the occurrence of generation of a carrier signal during transmission preparation by generating a transmission enable signal replying with a transmission request, a driver control signal and an unsuccessful notice signal on the occurrence of unsuccessful transmission.

CONSTITUTION: A CPU hardware 25 gives a transmission command to a communication LSI 26 and a driver control section 30 monitors detection of a carrier during a window period set by a CPU 25 for a preparation period before the LSI 26 makes a transmission request signal significant. Thus, the CPU 25 is quickly coped with for the transmission unsuccess on the occurrence of the carrier. Furthermore, a driver 27 is closed by the monitor and even when the carrier is caused for the transmission preparation period of the LSI 26, the transmission to a LAN bus S is stopped even on the occurrence of the carrier during the transmission preparation period of the LSI 26. Furthermore, in the case of unsuccessful transmission, a time by a preset flag is not reserved and the communication enable signal is significantly changed, then the time required for the processing of the LSI 26 is shorten.