To effectively prevent breakage of a container during heat generation of a heating agent containing powder that generates heat by addition of water.

This heating agent includes a heat-generating powder 3 capable of generating heat by addition of water, and a sealing container 5 in which the heat-generating powder is sealed. The sealing container is composed of water permeable woven fabric or nonwoven fabric including thermally adhesive fiber, and has a thermally-adhered part 7 formed by thermally adhering the woven fabrics or nonwoven fabrics. The thermally-adhered part is composed of an outer hull part 13 positioned at an outer hull and an inner hull part 15 positioned at an inner hull adjacent to and surrounded by the outer hull part. The outer hull part and the inner hull part are constituted in a state that the inside and outside of the sealed container is uncommunicatable as the inner hull part is separatable by vapor pressure generated by heat generation of the power generating heat by addition of water, and at least a part of the outer hull part is unseparatable.