To provide an insolubilizing agent for fluorine highly effective for reducing eluted fluorine in waste water treatment, sludge treatment and waste treatment, etc., in various industrial fields.

By using grid or lime mud containing calcium carbonate, calcium hydroxide, sodium carbonate and calcium phosphate prepared in a causticizing process in existing kraft pulp manufacture respectively independently or by mixing them, the insolubilizing agent for fluorine, which is mixed for use in drainage or moisture-containing waste, is obtained.