PURPOSE: To shorten a communication time and to improve reliability put on a line by reducing the transmission time of a preamble to a prescribed value when no error exists, and extending the transmission time when the error exists.

CONSTITUTION: The reception signal of an antenna 1 is demodulated with a receiver 2, and error detection is performed at a detection circuit 4 by utilizing an error correction code. A detection result is supplied to a microprocessor 3 and an error counter circuit 5 as error status. When the value exceeds the prescribed number of count of the error, a preamble length change instruction is issued to the processor 3. The processor 3 changes a transmission protocol, and transmission data in which preamble length is changed is transmitted via a transmitter 6. At such a case, the transmission time of the premable is reduced to the prescribed value, and that time of the preamble is extended until the error is eliminated when the error exists.