Abstract: Method and apparatus for removing extraneous matters from cooling water used in a tubular heat exchanger such as a condensor is disclosed. When a pressure difference between the portions upstream and downstream of a filter element exceeds a predetermined threshold value set in advance in accordance with the flow rate of the cooling water, a blow-down operation is started to remove the extraneous matters scraped off of the outer peripheral surface of the filter element by a turning flow. The extraneous matters are removed at a rate of clogging of the filter element of 50%, irrespective of the flow rate of the cooling water. The clogging rate can be sensed accurately, and the blow-down operation is effected with necessary and sufficient frequency.