To precisely grasp the heat value of a glass-solidified body by measuring the temperatures of a heat value measuring medium carried from a measuring vessel to a medium circulating means and the heat value measuring medium carried from the medium circulating means to the measuring vessel, respectively.

A glass-solidified body 6 is set into a measuring vessel 1, a heat value measuring water 7 is filled therein, the pump 9 is operated in the state where opening and closing valves 11, 12 are opened. The heat value measuring water 7 is run out from the measuring vessel 1, passed through a piping member 8, and continuously circulated to the measuring vessel 1 through a filter 10 and a heat exchanger 3. The flow rate of a heat exchange medium 13 to the heat exchanger 3 is regulated, and the temperature of the heat value measuring water 7 carried from the heat exchanger 3 into the measuring vessel 1 is made constant. The temperature of the heat value measuring water 7 carried from the measuring vessel 1 to a medium circulating means 2 and the temperature of the heat value measuring water 7 carried from the medium circulating means 2 to the measuring vessel 1 are detected by temperature detectors 14, 15, respectively. When both the temperatures are laid in steady state, the heat value of the glass-solidified body 6 is determined on the basis of them.