To provide a preliminary grinding method of a metallic plate for printing plate, which is capable of maintaining the quality of surface of the same nicely.

In the preliminary grinding method of the metallic plate W for printing plate wherein the surface of the metallic plate W for a running belt type printing plate is ground by buffing through a grinding roller 16 while circulating and supplying washing water, reserved in a circulating tank 22, on the surface of the metallic plate W for the running belt type printing plate, the concentration of solid matters contained in the washing water, circulated and supplied from the circulating tank 22, is specified so as to be 20 g/l or less while the concentration of metallic substances contained in the washing water is specified so as to be 15 g/l or less. According to this method, the quality of surface of the metallic plate W for printing, which is treated through working treatment, can be maintained so as to be higher than a given level.