A heating element (10) according to the present invention comprises a structure including a heating layer (11) that includes an oxidizable metal (21), a desiccant (22), and water, and a fiber layer (12) with a maximum water holding rate of 500% or less, wherein the content of water in the heating layer (11) is 10 to 60% by mass, and the hardness of the structure measured under the following condition is 10 to 200 g/mm. (Condition) A test piece cut from the structure so as to have a square surface with a side length of 2 cm and a cylindrical pipe with an inner diameter of 1 cm are prepared, and the test piece is arranged on the cylindrical pipe so that the central part of the square surface and the central part of the cylindrical pipe overlap. Then, using a cylindrical plunger with a diameter of 1 mm, a rupture test is performed by lowering the distal end of the plunger from the top of the test piece to the internal center of the cylindrical pipe at a speed of 5 cm/min. In this rupture test, the maximum value of the rupture stress until the test piece ruptures is detected and is treated as a (g). The hardness is found by dividing a by the average thickness b (mm) of the heating layer (11).本發明之發熱體(10)具備含有發熱層(11)與纖維層(12)之構造物,該發熱層(11)包含被氧化性金屬(21)、吸水劑(22)及水,該纖維層(12)之水之最大保持率為500%以下,且發熱層(11)中之水之含量為10質量%以上且60質量%以下,上述構造物於以下條件下測定之硬度為10g/mm以上且200g/mm以下。(條件)準備將上述構造物以具有邊長2cm之正方形之面的方式進行切割而得之試驗片、與內徑1cm之圓筒管,以上述正方形之面之中心部與上述圓筒管之中央部重合的方式於上述圓筒管上配置上述試驗片。繼而,使用直徑1mm之圓柱形狀之柱塞,使上述柱塞之前端以5cm/min之速度自上述試驗片之上部向上述圓筒管之內側中心下降,而進行破斷試驗。於該破斷試驗中,檢測直至上述試驗片破斷為止之破斷應力之最大值,將其設為a[g]。將該a除以發熱層(11)之平均厚度b[mm],藉此求出上述硬度。10‧‧‧發熱體11‧‧‧發熱層12‧‧‧纖維層21‧‧‧被氧化性金屬22‧‧‧吸水劑
