PURPOSE: To decrease the segmenting time of a cause of malfunction due to external noise or a fault of the hardware by connecting a D input terminal of a D type flip-flop to ground, connecting a clock input terminal via a resistor, detecting a noise pulse by a resistor, reading the voltage at a D input terminal by a pulse voltage so as to display the noise.

CONSTITUTION: When noise is generated, a current flowing through a resistor R1 2 from a clock CLK terminal of a flip-flop 1 is changed to change the potential at the CLK terminal, thereby bringing the level to a high level tentatively. Thus, the flip-flop 1 reads a low level at the D input terminal to bring the level of the Q output terminal to a low level. Thus, a current flows from a VCC to a light emitting diode 10 via a resistor R7 11 and various sensitivities are set for the noise detection sensitivity for lighting the light emitting diode by selecting a dip switch DIP SW6. Thus, whether a cause to a trouble of an electric device comes from external noise or an internal cause such as program bug is segmented easily.