PROBLEM TO BE SOLVED: To provide a filter for water bath which allows simple water drainage and a simple structure of a water tank.SOLUTION: The present invention relates to a filter for a water bath in which water W flows along a circulation flow by driving a circulation pump 2, the filter including: a filter casing 1 provided below a water level in a water bath A and in which filter medium 15a, 15b are stored; a circulation flow in which the water W in the water bath A flows into the filter casing 1, flows through the filter medium 15a, 15b, and flows out of the filter casing 1 into the water bath; and a circulation pump 2 provided on the circulation flow. Flow switching means is provided on a downstream side from the circulation pump 2 in the circulation flow. The flow switching means is configured to allow a switching operation between: a circulation state in which the water W flowing from an inflow port 42 is flows out of a circulation outlet 43 to circulate along the circulation flow; and a discharge st
