PURPOSE: To perform a test of a data transfer device to various types of faults in a simple way, by continuously suppressing or holding a signal which is transmitted the data transferring device from an input/output device or generating the signals other than the above-mentioned signal.

CONSTITUTION: A pseudo fault generator 3 contains a means 36 which suppresses in the course of a signal transmitted to a data transferring device 1 from an input/output device 2, and also has a holding means 37 which transmits continuously the signal given from the device 2 although said signal is reset by the device 2. In addition, a signal switching means 38 is provided to produce the signals other than the signal given from the device 2. A connection switching means 39 switch the signal lines among the means 36W38, the device 2 and the device 1. Thus a test is performed in a simple way for the data transfer device to various types of faults.