PURPOSE: To detect an abnormality of a serial transfer line automatically by connecting a 2nd set of serial/parallel conversion means to a 1st set of serial/ parallel conversion means serially and connecting an output of the 2nd set of serial/parallel conversion means to a parallel data line via a parallel data output means.

CONSTITUTION: When a data output control means 1 outputs a test data to a parallel/serial control means 4 to transfer the data serially to the 2nd set of serial/parallel conversion means 11 through the 1st set of serial/parallel conversion means 71-74 serially, the test data is outputted in parallel from the conversion means 11 when the serial transfer line is normal. If there is a broken, short-circuit or faulty connection serial transfer line, the conversion means 11 outputs no test data. Then the parallel data output means 12 outputs the parallel output data from the conversion means 11 to signal lines I0-I7, the data output control means 1 reads the data via the signal lines I0-I7 to detect the propriety between the read data and the test data thereby detecting the serial transfer error.