To provide a filtering method of a used grinding liquid requiring no much labor, reducing running cost, removing even fine particle-like chips, improving finishing accuracy of a grinding surface, and improving product accuracy.

This filtering method of the used grinding liquid is provided for removing nonferrous metal grinding chips by a magnet together with ferrous grinding chips by passing the mixed grinding liquid through a magnet type grinding chip separator by mixing the ferrous grinding chips in the grinding liquid mixed with the ground nonferrous metal chips. A mixing quantity of the ferrous grinding chips is desirably set to 300 grams to 1 liter of the grinding liquid mixed with the nonferrous metal grinding chips.