A method for removing phosphorus is provided in which eutrophic water as a treatment target is supplied to fly ash thereby removing phosphorus contained in the eutrophic water. The fly ash includes about 3% or more calcium oxide as a component. Alternatively, the fly ash consists of a double-structure embedding fine particles in a cavity of a hollow-particle structure having a hole in a part of the surface. Alternatively, the fly ash includes about 3% or more calcium oxide as a component and consisting of a double structure embedding fine particles in a cavity of a hollow-particle structure having a hole in a part of the surface.