To realize an inspecting method capable of accurately inspecting the reliability of a polycrystalline silicon membrane transistor.

The source and drain of a polycrystalline silicon membrane transistor 10b provided in a liquid crystal display are grounded, and a positive voltage of 10 V or higher is impressed to the gate. The polycrystalline silicon membrane transistor 10b is simultaneously maintained at temperatures equal to room temperature or higher for a predetermined time to impress stress. The amount of fluctuation in the threshold values of the polycrystalline silicon membrane transistor 10b prior to and after the impression of such voltage and temperature stress is measured, and its reliability is determined according to the amount of fluctuation in the measured threshold values.