The present invention relates to a heating element (1) having an exothermic composition containing an oxidizable metal, a carbon component and water. Tri-alkali metal phosphate is contained in the heating element (1), and the content of water in the heating element (1) is equal to or larger than 50 parts by mass and equal to or smaller than 90 parts by mass for 100 parts by mass of the oxidizable metal, and the content of tri-alkali metal phosphate as a phosphate group is equal to or larger than 0.5 parts by mass and equal to or smaller than 1.1 parts by mass for 100 parts by mass of the oxidizable metal.
