PURPOSE: To perform easily an automatic diagnosis of an interface part, by generating a false signal corresponding to a signal generated by an external input device on the basis of data which is programmed and stored in advance.

CONSTITUTION: A control circuit 6 performs control so that a data selecting circuit 10 selects a false keyboard signal 8. Then, a data generating circuit 7 is instructed to output data. The circuit 7 outputs the data programmed therein. The false keyboard signal is composed of this data and a strobe signal from a circuit 6, held by a data latch circuit 11 through the circuit 10, and inputted to the internal arithmetic mechanism of a terminal device. Then, processing is performed to check data sent to a host compouter system finally. Consequently, the diagnostic system with high reliability is obtained.