PURPOSE: To prevent a heat accumulating member from being damaged by ascending or dropping of the heat accumulating member by a method wherein a degree of opening of a pressure adjusting valve is set to such a degree as one in which the heat accumulated member stored within a heater does not float up even if a rapid change occurs at pressure within the heater.

CONSTITUTION: At a control device 10, a flow rate coefficient CV is calculated by applying a pressure P2 in a heater 1 to be transmitted; an upstream side pressure P1 of a pressure control valve 5; an air temperature T, a flow rate W in which the heat accumulated member 3 starts to float up got by performing a test in advance after being inputted; characteristics data of the pressure adjusting valve 5; and data about nature of air to be applied and the like. An upper limit degree of opening of a limit where the heat accumulating member 3 floats up at the pressure adjusting valve is calculated under application of the calculated flow rate coefficient CV. Then, a degree of opening instruction for setting a degree of opening of the pressure adjusting valve 5 to a limit value not exceeding the upper limit degree of opening is given. Accordingly, the heat accumulating member 3 mounted on a grid is not always floated up within the heater 1, its damage is prevented, thereby a shutting-off valve 9 or a test piece at a downstream side of the heat accumulating member can be prevented.