A method of removing impurities in a heat-transfer tube group in a power device or a power-generating device which uses a Stirling engine, comprising: a displacer piston a power piston, a high temperature space, and a low temperature space, wherein the high temperature space and the low temperature space are separated by the displacer piston, a heating portion disposed at a position opposed to the displacer piston across the high temperature space, a regeneration portion and a cooling portion are disposed on an outer periphery of the displacer piston, heat-transfer tubes configuring the heating portion disposed in a flue, the method comprises: heating and expanding an operation gas in the heat-transfer tubes, cooling and contracting the operation gas in the cooling portion, and moving the operation gas between the high temperature space and the low temperature space, wherein a removing mode of impurities included in exhaust gas which flows through the flue is carried out when the impurities adhere to the heat-transfer tubes, characterized by reducing the number of revolutions of the Stirling engine without shutdown of an operation of the Sterling engine, reducing an amount of generated electricity at the Stirling engine without shutdown of an operation of the Sterling engine, or reversibly rotating the Stirling engine, and raising a temperature at the heat-transfer tubes so that the impurities which adhere to the heat-transfer tubes can be peeled off. a displacer piston a power piston, a high temperature space, and a low temperature space, wherein the high temperature space and the low temperature space are separated by the displacer piston, a heating portion disposed at a position opposed to the displacer piston across the high temperature space, a regeneration portion and a cooling portion are disposed on an outer periphery of the displacer piston, heat-transfer tubes configuring the heating portion disposed in a flue, the method comprises: heating and expanding an operation gas in the heat-transfer tubes, cooling and contracting the operation gas in the cooling portion, and moving the operation gas between the high temperature space and the low temperature space, wherein a removing mode of impurities included in exhaust gas which flows through the flue is carried out when the impurities adhere to the heat-transfer tubes, characterized by reducing the number of revolutions of the Stirling engine without shutdown of an operation of the Sterling engine, reducing an amount of generated electricity at the Stirling engine without shutdown of an operation of the Sterling engine, or reversibly rotating the Stirling engine, and raising a temperature at the heat-transfer tubes so that the impurities which adhere to the heat-transfer tubes can be peeled off. heating and expanding an operation gas in the heat-transfer tubes, cooling and contracting the operation gas in the cooling portion, and moving the operation gas between the high temperature space and the low temperature space, wherein a removing mode of impurities included in exhaust gas which flows through the flue is carried out when the impurities adhere to the heat-transfer tubes, characterized by reducing the number of revolutions of the Stirling engine without shutdown of an operation of the Sterling engine, reducing an amount of generated electricity at the Stirling engine without shutdown of an operation of the Sterling engine, or reversibly rotating the Stirling engine, and raising a temperature at the heat-transfer tubes so that the impurities which adhere to the heat-transfer tubes can be peeled off.