To shorten the shutdown time of a filtration apparatus for circulating cooling water due to replacement of a filter etc. to prevent lowering in productivity as a result by restraining the clogging of the filter thereby maintaining the filtering function for a long period of time.

The filtration apparatus is constituted in such a way that the filtration surface of a communicating and freely windable belt-like filter medium 4 having a spare rolled filter medium part which is not immersed in circulating cooling water is immersed in the circulating cooling water stored in a water storage tank 2 and the communication part is connected to a pump 3 for sucking the stored circulating cooling water. Since the stored circulating cooling water is sucked by the pump 3 through the belt-like filter medium 4, scale is not contained in the circulating cooling water flowing in a metallic mold 1 and scale is not stuck to/accumulated in a cooling water hole of the metallic die 1, so that the narrowness or blockade of the cooling water hole owing to the exfoliation of scale can be prevented.