By using the method of grinding slot nozzles of the present invention, the gapping place between a slot nozzle and a back roller is basically constant relative to the whole width of the slot nozzle. The method of grinding slot nozzles comprises the first process of grinding the front edge part of the slot nozzle by a grinding device to make the straightness of each meter in the length direction of the front edge part within 200 micron under the room temperature, wherein, the slot nozzle sprays out fluid from a narrow slot formed by folding at least two blocks, the second process of making a roller prepared additionally rotate by a rotary mechanism, wherein, the roller having a circular degree within 3 micron and a face length (L2) longer than the length (L1) of the slot nozzle is provided with a grinding mechanism on the outer circumference surface, and the third process of pressing the rotating roller onto the front edge part of the slot nozzle for grinding the front edge part.