Disclosed is a heat insulator comprising a substrate comprising of a bulk of silica-based inorganic fiber containing a hydroxyl group; a metallic or ceramic infrared mediator held on at least a part of one surface of the substrate; and a silica cured product holding the infrared mediator on/in the substrate. As the infrared mediator, a metal foil or a ceramic particle may be used. This heat insulator exhibits excellent heat insulating performance in a high temperature range of 600° C. or more, and can be molded into a three-dimensional shape which can be directly mounted to a structure.