PURPOSE: To make it possible to reduce the quantity of diagnosis data by converting only varied parts of the preceding test and expected output pattern into data, and restoring to the original and resetting the expected output pattern by an expected output pattern setting part.

CONSTITUTION: When a diagnosing input pattern is imparted from a memory device 1 to a part 2 to be diagnosed, the output pattern of the part to be diagnosed appears at one input side of a comparison part 3. On the other hand, with respect to the first test, the expected output pattern is directly set at each of memory elements corresponding to each of output terminals within the expected output pattern setting part 4 from the memory deviced 1 with respect to the full output terminal, and is compared 3 with the output from the part 2 to be diagnosed. With respect to the second test and downward, the expected output pattern of the preceding test is housed in the memory element of the expected output pattern setting part 4. Since the expected output pattern obtained from the memory device 1 is an information showing a position different from the preceding test, only the content of the designated memory element in the expected output pattern setting part 4 is reset by the data obtained.