A noise signal for a malfunction test is inputted to a ground terminal, and a frequency property of the magnitude of the power at which a designated electric circuit causes a malfunction is found. A detection reference ground of a detection part for detecting malfunctions of the designated electric circuit is connected at high impedance to a ground of the targeted circuit. Differential input parts are provided to the detection part, a to-be-detected part of the designated electric circuit is connected to one differential input part, and the ground of the targeted circuit is connected to the other differential input part. A ground of a noise source is isolated from a ground of a power source circuit for supplying a power source to the designated electric circuit. The ground of the noise source is galvanically isolated from the ground of the designated electric circuit.
