A machining quality judging method for a wafer grinding machine and wafer grinding machine are disclosed. The thickness of a wafer 2 is acquired from the feed amount of a grinding unit 3 while at the same time actually measuring the thickness of the wafer 2 appropriately. The wafer grinding machine includes a machining quality judging unit 20 for comparing the thickness of the wafer 2 based on the feed amount of the grinding unit 3 with the actually measured thickness of the wafer 2 and judges the machining quality of the ground surface of the wafer 2. Upon judgment of a machining failure, a command is issued to stop the back surface grinding operation.