In the case where a possible defect of a liquid crystal panel is detected, after the liquid crystal panel is put into an oven with the liquid crystal panel being energized, a second inspecting pulse, which has a larger potential difference than a potential difference of a first inspecting pulse applied to a second signal line on an active matrix substrate, is applied to a second signal line. In the above method, since an insulating layer, which is on the verge of breakage, between a source line and the second signal line can be broken, a possible defect of the liquid crystal panel can be detected as a cross bright line by inspection for turning-on in the panel inspecting step. For this reason, in this method, accuracy of detecting a possible defect can be improved, and the number of S-G leaks in the market is decreased greatly, thereby improving display quality of the liquid crystal panel.