A warming device of sheet form having (1) a heat generating sheet prepared by papermaking and containing an oxidizable metal, a moisture retaining agent, and a fibrous material and (2) an air permeable holder holding the heat generating sheet. The warming device has a thickness of 0.1 to 10 mm and a flexural strength of 0.01 to 0.3 N/cm. The sheet preferably has a thickness of 0.1 to 2.0 mm. The fibrous material preferably has a CSF of 600 ml or less.