PURPOSE: To enable to control the temperature of a heating mat of the type utilizing heat produced by a reaction between an oxidizing agent and a reducing agent, by using a diffusion controlling film capable of passing at least one of the oxidizing and reducing agents, and charging them in a bag-like body in the manner that the oxidizing and reducing agents are separated from each other by the intermediary of said diffusion controlling film.

CONSTITUTION: A heating mat of this invention is obtained by preparing a small bag- like member by enveloping an oxidizing agent (or a reducing agent) 4 with a diffusion controlling film 2 and covering the same with an impermeable film 3, putting the small bag-like member into a large bag-like member 1, and charging a reducing agent (or an oxidizing agent) 5 in a hollow space between the small and the large bag-like members. Here, the diffusion controlling film 2 is made, for instance, of papers such as Japanese paper or fabrics of natural or synthetic fibers which is capable of passing the oxidizing agent (or reducing agent) therethrough but is not ruptured easily even if an external pressure is acted thereto during use. Further, the impermeable film 3 is made of a material which is capable of preventing the content of the small bag-like member from passing therethrough and can be ruptured with ease by applying an external force in using the heating mat.