To provide a method for judging the dressing timing of a grinding wheel during grinding a workpiece.

The workpiece is ground by the relative movement of the workpiece 2 laid on a laterally movable X-axis table 4 and the grinding wheel 3, which is movable in the forward/backward directions (the Y-axis direction) and held to a wheel head 10 provided so as to ascend/descend in the vertical direction (the Z-axis direction) in the grinding device 1. The dressing timing of the grinding wheel during grinding while supplying a grinding liquid by a nozzle 30 toward the grinding point, where the grinding wheel and the workpiece come into contact with each other, is judged by using the grinding device 1 in a following manner. It is decided as the dressing timing of the grinding wheel when a color temperature, which appears on thermograph paper 20 pasted to the front of a grinding wheel protection cover 17, reaches the color temperature range determined to reach the dressing timing by experiments beforehand.