PURPOSE: To correct a transmission error automatically without adding error correcting information by detecting its transmission error at reception side when a transmission error takes place and retransmitting the transmitted data from the transmission side.

CONSTITUTION: After a parallel conversion signal 16 is outputted, if a normality reception confirming signal 17 is not returned from a serial input controller 7 in the specific time, a serial output controller 4 discriminates it as the occurrence of a transmission error. Then the serial data shift clock 15 is outputted again and the serial data 14 is sent between the parallel input serial output shift register 3 with a serial input and a serial input/parallel output shift register 5. The serial data 14 in this case is the same data as the preceding data. Thus, the data sent again is received as a received data 18 by a reception side host processor interface 8 only when no transmission error takes place by the parity error signal and the transmission error is corrected automatically.