Abstract: A container capable of heating the contents of the container from inside, having at its bottom a cylindrical inner shell made of a thermoconductive material opening downward and defining internally of itself a reaction chamber in which are disposed an active substance and a reacting substance capable of causing an exothermic reaction upon contact with each other. These substances are isolated from each other by a partition. A sealing lid is disposed across the opening of the inner shell so as to seal the reaction chamber and constitute the container bottom plate, and has a temporarily sealed piercing hole. A piercer is used for piercing through the temporarily sealed piercing hole into the reaction chamber so as to rupture the partition isolating the reaction components to cause contact of the reaction components with each other to initiate the exothermic reaction.