PURPOSE: To efficiently perform transmission by providing an auxiliary start bit transmitting means and a supplementary data transmitting means to prevent the occurrence of synchronization slip and ineffective data even in the case of impossibility of data transmission due to abnormality in a device transmitting data at present.

CONSTITUTION: An auxiliary start bit transmitting means 3 generates an auxiliary start bit signal synchronously with the start bit signal transmitted by a transmitter 1. A transmitting operation stop detecting means 4 detects whether the transmitting operation of the transmitter 1 is stopped or not, and a supplementary data transmitting means 5 outputs supplementary data corresponding to a data part, which is not transmitted, in the frame by the transmitting operation stop detection signal of the means 4. If the transmitter which transmits data cannot transmit data because of abnormality, the auxiliary start bit is preliminarily transmitted synchronously with the transmitter to prevent interruption of a text, and deficient characters are transmitted. Thus, the unnecessary synchronization restoring processing or frames are eliminated to efficiently transmit data.