An object of the present invention is to provide an exothermic device which is capable of reaching a high temperature upon use, and capable of sustaining a high temperature state for a prolonged period of time. The present invention provides an exothermic device (1) including: an exothermic agent (5)； and a packaging material enclosing the exothermic agent (5). The packaging material is in the form of a flat bag composed of a first surface and a second surface. Each of the first surface and the second surface includes a breathable region (2) and a non-breathable region (3).
