PURPOSE: To provide a circulating filter a filter in which medium is harder to break due to impact, etc., and the quantity of the filter medium flowing out together with filtered water is extremely small and the washing effect of a filter bed is higher and running cost is lower, compared to a circulating filter using sand as a filter medium.

CONSTITUTION: A pressure vessel 101 is divided into two parts, a filter part 107 and a filtered water collecting part 108 by a filter bed plate 106. In the filter part 107, a filter bed 130 consisting of a sand-like diatomaceous earth sintered filter medium 131 and a space 132 above the filter bed 130 is formed. An opening 143 of the 1st pipe 141 is arranged in the upper position in the space 132. A strainer 120 having plural slits 124 of a width smaller than a particle diameter of particles having a minimum particle diameter in the diatomaceous earth sintered filter medium 131 is fitted to the filter bed plate 106 and the filter part 107 and the filtered water collecting part 108 are made communicate with each other though the strainer 120. The 2nd pipe 142 is installed at the bottom part of the pressure vessel 101 to communicate the filtered water collecting part 108 with the inside of the 2nd pipe 142.