A heat generating element of the present invention contains an oxidizable metal, a moisture retaining agent, water, an electrolyte as an oxidation promoter, and a particulate hardening inhibitor. The particle size of the particulate hardening inhibitor is preferably 25% or smaller of that of the oxidizable metal. The heat generating element in which the oxidizable metal to the particulate hardening inhibitor mass ratio is preferably 1 to 30. The particulate hardening inhibitor preferably has a particle size of 10 µm or smaller.
