an outer container which contains a heating element generating heat by reaction with water and has a first extension part extending outwards from the edge of one side and an inflow passage formed inside the first extension part so that water flows into the outer container; an inner container which contains foods, has a shape corresponding to the outer container to be combined with an upper side of the outer container, and has a second extension part formed at the edge of one side to be coupled with the first extension part of the outer container in order to close the inflow passage; and a cover combined with an upper portion of the inner container,

wherein the outer container has a coupling rib protruding upwards along the edge of the upper side and the inner container has a fitting groove formed along the edge of the upper side in such a way that the coupling rib is insertion-coupled with the fitting groove, so that the outer container and the inner container are coupled with each other in an insertion type, and

wherein the first extension part has a plate-type flange formed at the edge portion and the intermediate part of the first extension part becomes a part of the inflow passage, the second extension part is formed in a plate-type flange, the first extension part of the outer container has a coupling protrusion protruding upwards from an end portion, and the second extension part of the inner container has a coupling groove formed at an end portion so that the coupling protrusion is insertion-coupled with the coupling groove.