It is an object of the present invention to provide a chemical body warmer that achieves a sufficient heating effect even in a low-temperature environment at about 10° C. or less. A chemical body warmer including a flat housing including one or a plurality of cells each containing an exothermic composition that generates heat upon contact with air, wherein a total area of the cell or the plurality of cells in a plan view of the housing is 200 cm2 or more, and the chemical body warmer is used by being placed under a blanket.