To dispense with back washing work, to allow a dissolved organic matter to be adsorbed and decomposed and to enhance the filtering capacity of a filter apparatus 10, in the circulating filter system of a large-sized bathtub for circulating hot water in the bathtub 1 by a circulating passage 2 and filtering this circulated water by the filter apparatus 10 to return the filtered water to the bathtub 1.

A filter for capturing impurities in circulated water is provided replaceably and an activated stone tank for adsorbing organic matter dissolved in the circulated water is disposed to constitute the filter apparatus 10. An oxidizing agent is injected in the circulating passage 2 on the primary side of the filter apparatus 10 while a concentration sensor 9 is provided on the secondary side of the filter apparatus 10 and the injection amount of an oxidizing agent is controlled on the basis of the detection signal of the concentration sensor 9.