A heating element (10) is obtained by laminating a heat generation layer (11), which contains an oxidizable metal, a water absorbent and water, and a water holding layer (12), which is formed of a water absorbent sheet (102). The content of the water absorbent is 0.3-20 parts by mass relative to 100 parts by mass of the oxidizable metal. The mass ratio of the water content to the content of the water absorbent in the heat generation layer (11), namely (water/water absorbent) is 0.8-13. The water content in the water holding layer (12) is 10-45% by mass of the maximum water absorption amount of the water holding 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
