To easily discriminate what causes failure of display irregularity in the case of inspecting picture quality of a liquid crystal panel.

The end parts of picture signal lines 8a, 8b, 8c,..., 8m for supplying video signals to signal lines 9-1a, 9-1b, etc., divided into a plurality of phases are connected with analog switches 14a, 14b, 14c, etc., for short-circuiting all the picture signal lines and equalizing each signal line potential to be on the same level as a signal line inter-short-circuiting circuit 100. By performing the picture quality inspection by equalizing the output potentials of all the signal lines to be on the same level, it is discriminated whether the display irregularity is caused by variation in outputs of the divided signal line phases due to external circuits or it is caused by the characteristic of the liquid crystal panel.