PURPOSE: To always maintain a glass material at suitable temperature range and to prevent generation of defective product by detecting the temp. of heated part of a glass material with an infrared ray sensor and controlling the heating temp. by forecasting the heating temp. at the end stage of heating from the level of detected temp.

CONSTITUTION: A signal In from an index signal generator 4 is transmitted through a synchronous signal generator 5 to a sample-and-hold circuit 3 and a signal (r) from a reset signal generator 6 is transmitted simultaneously to the circuit 3. The temp. condition of the heated part of the glass material in the midway of heating is detected by the infrared ray sensor 1, and the signal is transmitted through a main amplifier 2 to the circuit 3. The signal H from the circuit 3 fed to a temp. level discriminating circuit 7 and with the suitable temp. range after the end of heating with a circuit 7. If the temp. of the glass material is discriminated to exceed the suitable temp. range at the end of heating, an instruction is given to a driving device 14 to perform air-cooling, and if the temp. of the glass material is discriminated to become lower beyond the suitable temp. range at the end of heating, an instruction is given to a heating power control device 15 to enhance heating.