There are provided a process for producing an active heat generating mixture capable of causing an oxidation reaction immediately upon contact with air to generate heat and subsequently initiating an abrupt reaction which after arrival at a certain fixed temperature, is then changed to a mild reaction, the heating generating mixture being cheap and being able to be industrially mass-produced; a heat generating mixture; a heat generating composition; a moldable heat generating composition; and a heat generating body. The invention is characterized by bringing a reaction mixture containing, as essential components, an iron powder, a reaction accelerator and water and having a water content of from 0.5 to 20% by weight and a water mobility value showing an amount of surplus water of less than 0.01 with an oxidizing gas under circumstances at 0.degree. C. or higher and regulating a temperature rise of the reaction mixture at 1.degree. C. or higher within 10 minutes.