The present invention provides a sustained-releasing anti-protista preparation, including using a water-insoluble or difficulty water soluble and water-wettable polymer compound which is solid at room temperature as a sustained-releasing substrate material, a method for a preparation of the sustained-releasing anti-protista preparation, including kneading a water-insoluble or difficulty water soluble and water-wettable polymer compound which is solid at room temperature with an anti-protista substrate, and a sustained-releasing anti-protista preparation for a waterway, including using a water insoluble or difficulty water soluble and water-wettable polymer compound which is solid at room temperature as a sustained-releasing substrate material.