Heating elements (10) of the present invention, the oxidizable metal (21), the water-absorbing agent (22) and the heating layer containing water (11) and the fiber layer up to the retention of water is less than 500% (12) If, comprising a containing structure, content of water in the heat generating layer (11) is 10 mass% or more and 60 mass% or less, of the structure, the hardness measured under the following conditions 10 g / it is equal to or less than mm or more 200g / mm.
(conditions)
A test piece cut the structure to have a square face of the side 2 cm, prepared cylindrical tube having an inner diameter of 1 cm, and the central portion of the surface of the square, as the central portion of the cylindrical tube overlap , placing the test piece on the cylindrical tube.
Then, using a plunger cylindrical shape 1mm diameter, the plunger tip from the top of the test piece inside the center of the cylindrical tube is lowered at a rate of 5 cm / min, performing rupture test.
At this rupture test, wherein the test piece detects the maximum value of the breaking stress until breakage, which is referred to as a [g].
By dividing the a in average thickness b [mm] of the heating layer (11), determining the hardness.