A simulator for simulating a digital circuit, includes an input circuit for inputting a test patterns to describe the characteristics of the digital circuit and for inputting input signals to test the operation of digital circuit and output signals to describe the expected output of the digital circuit based on the input signals, an applying circuit to apply the input signal to test the operation of the digital circuit to the test patterns to describe the characteristics of the digital circuit to form a simulated output signal to indicate a response based on the test pattern, a comparator circuit to compare the simulated output signal with the output signal to describe the expected output of the digital circuit based on the input signals to determine a difference between the simulated output signal and the output signals, wherein the operation of the simulation is stopped if the difference is greater than a predetermined difference.