To detect a defect as failure to grind at the peripheral edge of a glass sheet, produced due to variations in the pressing force of a grinding wheel generated, when advancing in a skewed attitude or running in meandering.

The method concerns monitoring a grinding failure through the current flowing in a grinding motor, when the left and right side edges of a glass sheet moving in transit are ground by a grinding wheel, with which, when grinding the side faces of the glass sheet is conducted, while the presence of the glass sheet is detected from the glass sheet forefront to the tail by using a detector, in an arrangement such that the grinding wheel is pressed against the side edges of the glass sheet, detecting the failure to grind of the side edges is facilitated through a procedure such that two current waveforms, exhibiting the trend of the current flowing in drive motors for the left and right grinding wheels, are displayed on a display device in an arrangement that are overlapped on the same time axis.