PURPOSE: To automatically detect a fault of a serial transmission line by connecting a shift output terminal of a serial/parallel conversion means to a data output control means controlling data input/output of a parallel/serial conversion means.

CONSTITUTION: A shift output terminal (SPH of 7n) of serial/parallel conversion means 71-7n connects to an input port P0 of a data output control means 1. The data output control means 1 outputs (1st time) a test data of a serial input bit number of the serial/parallel conversion means 71- 7n to a parallel/serial conversion means 4, inputs it to the serial/parallel conversion means 71-7n in serial shift form and outputs (2nd times) the data of a bit number equal to a bit number of the test data again to the parallel/serial conversion means 4 and then inputs it to the serial/parallel conversion means 71-7n in serial shift form, reads the serial shift output (SQH of 7n) of the serial/parallel conversion means 71-7n during that time, detects the propriety between the read data and the test data thereby detecting serial transfer error.