The viscosity stability in forming a slurry of one or metal components selected from water-insoluble metal oxides and metal salts is improved, and using the resultant slurry, a coating layer is formed on a polyolefinic resin porous film to give a multilayer porous film having excellent surface smoothness. The multilayer porous film has, as layered on at least one surface of a polyolefinic resin porous film, a coating layer that contains one or more metal components selected from water-insoluble metal oxides and metal salts, and a resin binder, wherein the content of water-soluble calcium contained in the metal components is 3 ppm by mass or less.