A detector substrate is provided adjacent to an element substrate to inspect any defect in a semiconductor element and a wiring formed on the element substrate. An electromagnetic wave is irradiated to a gas provided between the detector substrate and the element substrate to ionize the gas. An electric current between the detection substrate and the element substrate through the ionized gas is measured by an ammeter for a video signal for displaying white. Also, an electric current between the detection substrate and the element substrate through the ionized gas is measured by an ammeter for a video signal for displaying black. A ratio of these electric currents is obtained as a ratio of white and black. An element substrate having quality lower than a reference quality is removed as defective.