To solve the problem that in a cooling water filter disposed with a water quality preserving agent used for a cooling water system of an internal combustion engine, algae emerge by growth of microorganisms like fungi caused by corrosion and adhering of water scale, resulting in lowering of water quality preservation effect or lowering of a service life by clogging of the filter.

This cooling water filter 20 is formed by: making a filter medium carrying silver into pleats; joining both ends thereof to form a cylinder to form a filtering body 2; disposing and fixing end plates 5, 6 at the upper and lower ends to form a filter element 1; forming a cooling water filter element 10 disposed with the water quality preserving agent 7 in the filter element 1; disposing the cooling water filter element 10 in a vessel 15 having an inlet 12c and an outlet 12d; and further disposing a magnet 8 having a lot of holes in the filter element 1.

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