A heating element (10) comprises an exothermic layer (11) containing an oxidizable metal, a water absorption agent and water and a water-retention layer (12) having a water absorption sheet (102), the exothermic layer (11) and the water-retention layer (12) are in layers, in which the mass ratio of the content of the water absorption agent is from 0.3 to 20 parts by mass for 100 parts by mass of the oxidizable metal, mass ratio of content of water to the content of the water absorption agent in the exothermic layer (11) (water/water absorption agent) is from 0.8 to 13, and the content of water contained in the water-retention layer (12) is from 10 to 45 mass % of the maximum water absorption of said water-retention layer (12).本發明之發熱體(10)係將含有被氧化性金屬、吸水劑及水之發熱層(11)、以及由吸水片材(102)形成之保水層(12)積層而成者,且吸水劑之含量相對於被氧化性金屬100質量份為0.3~20質量份,於發熱層(11)中,水之含量相對於吸水劑之含量之質量比(水/吸水劑)為0.8~13,且保水層(12)中所含之水之含量為保水層(12)之最大吸水量的10~45質量%。10‧‧‧發熱體11‧‧‧發熱層12‧‧‧保水層102‧‧‧吸水片材(a)‧‧‧纖維材料(A)‧‧‧被氧化性金屬(b)‧‧‧吸水性聚合物(B)‧‧‧吸水劑
