it is the primary object of the present invention to provide a simple and accurate testing circuit and a testing method while occupying as small space as possible in an image display device . the testing circuit including a nand circuit connected in series is mounted on the image display device . a broken wiring on a data signal line and a defect in a data latch circuit can be detected by observing an output waveform from the testing circuit . accordingly , a broken wiring or the like on the data signal line and a scanning line and a defect in the latch circuit can be tested simply and accurately without an expensive testing apparatus and a great deal of time while occupying as small space as possible .