PURPOSE: To avoid the occurrence of such a case that a defectless product is wrongly discriminated as a defective product due to a phase difference in an inputted test pattern caused by the wiring length difference, etc., of a tester.

CONSTITUTION: A pattern generator 11 generates a test pattern A to be inputted to an IC 100 to be measured and an expected value pattern C when the pattern A is given to the IC 100. A pattern comparing/acceptance discriminating section 12 discriminates the acceptance of the IC 100 by comparing a pattern B outputted from the IC 100 and the pattern C outputted from the generator 11. When a discriminated result is 'no', a timing changing section 14 instructs the generator 11 new timing and tests are again performed at the new timing. These operations are continued until the IC is discriminated as defectless and the original functional tests are performed at the timing the IC 100 is discriminated as defectless.