PURPOSE: To prevent the occulusion of a water path, by providing a filter wherein a heat exchanger is connected to a closed pipeline for recirculating pure water for cooling electric machinery by a pump and a pH controller 8 for injecting alkali is connected to the aforementioned pipeline in the upstream side of said heat exchanger.

CONSTITUTION: An alkaline chemical solution is injected in the pipeline 2d from a pH controller 8 to adjust the pH of pure water to 8 and colloidal impurities in pure water are flocculated to precipitate particles of impurities with a large particle size. Therefore, the particles of impurities are collected and removed by a filter 5 and there is no possibility for imparting adverse influence on electric machinery 1. On the other hand, when alkaline pure water is supplied to the electric machinery as it is, there is possibility for damaging the electric machinery 1 because of the alkalinity thereof but there is no anxiety because the pH value of pure water is returned in the vicinity of 7 by passing pure water through an ion exchange resin tower 6.