PURPOSE: To easily and surely diagnose the disorder of the processing of each part by diagnosing the operation of an information data input part and an information data output part by the error detected result, the flag information, and the information data of the information data input part.

CONSTITUTION: The information data output part 2 adds the flag information CWD to a desired scctor address SADR by a P/S conversion circuit 8, and attaches an error detection code CRC to it through a CRC generation circuit 9, and modulates it, and outputs it to a transmission system 4 as transmitted data S1. The information data input part 3 demodulates the transmitted data S2 from the transmission system 4, and detects 16 an error by using the code CRC, and outputs the information CWD and the data SADR. Then a transmitted data looping-back part 13 which inputs the data S1 outputted from the output part 2 to the input part 3 as occasion demands is provided. In a state that the data S1 from the output part 2 is inputted to the input part 3, the operation of the output part 2 and the input part 3 is diagnosed by the error detected result D4, the flag information D5, and the information data D7, and the disorder of the processing of each part can be diagnosed.