NOVELTY - The method involves performing gas circulation process of allowing the flow of gas (g0) having a temperature more than the operating temperature of the solid-state-chemistry thermal storage material (31) in a U shaped heat exchange pipe (32) currently penetrated in a thermal storage tank (30) which stores a solid-state-chemistry thermal storage material. The inlet port and exit of the gas is reversed within the thermal storage tank of the heat exchange pipe by the switching process. The gas is a waste gas from the heat source equipment.    USE - Thermal storage method of thermal accumulator used in heat storage system such as cogeneration system for electric power generation apparatus such as gas engine, gas turbine, diesel engine and fuel cell.    ADVANTAGE - The heat can be thermally stored to the solid-state chemistry thermal storage material efficiently. The structure of piping for switch, and the operation valve can be simplified. The solid-state-chemistry thermal storage material can be heated efficiently.    DETAILED DESCRIPTION - The temperature of the solid-state-chemistry thermal storage material in the portion adjacent to the way point of the inlet port and exit is measured. The switching process is performed, when the temperature measured in the temperature-measurement process is more than the predetermined temperature, after starting the gas circulation process. INDEPENDENT CLAIMS are included for the following:    (1) a thermal accumulator has a thermal storage tank that is provided to store the solid-state-chemistry thermal storage material. The heat exchange pipe is currently penetrated in the thermal storage tank. A gas circulation unit (33) is provided to flow the gas having a temperature more than the operating temperature of the solid-state-chemistry thermal storage material in the heat exchange pipe. A switch unit is provided to reverse the inlet port and exit of the gas within the thermal storage tank of the heat exchange pipe; and    (2) a heat storage system has a heat source equipment from which the waste gas is ejected.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the thermal storage tank in the heat storage system.    Thermal storage tank (30)    Solid-state-chemistry thermal storage material (31)    Heat exchange pipe (32)    Gas circulation unit (33)    Gas (g0)