PURPOSE: A heat-evolving composition that is made by adding organic heat-storing substances such as higher fatty acid or animal or vegetable oil to a heat-evolving composition consisting of an iron powder, water and reaction accelerator, thus maintaining a constant temperature for a long time and being used suitably to heat human bodies very safely and simply.

CONSTITUTION: (A) a heat-evolving composition consisting of an iron powder of 100W300 mesh particle size, preferably 30W200wt% of water and 5W50wt% of a reaction accelerator such as sodium chloride or sodium sulfate on the basis of the iron powder, is combined with (B) an organic heat-storing substance prefereably melting at 30W90°C of a higher fatty acid such as lauric acid, higher alcohol such as myristyl alcohol or paraffin of 300W500 molecular weight to give the objective heat- evolving composition.