To provide a method and a device for improving the persistence to a thermal-insulating effect of an existing exothermic thermal-insulator by preventing damaging of the thermal-insulating effect caused by promotion of cooling with the exposure of the outer air by expanding the exothermic thermal-insulator and by restraining the cooling with the outer air to the expanded exothermic thermal-insulator.

A thermal-insulating box 8 has larger diameter than the inner diameter at the upper end of a feeder head brick 3 in a mold 1 for ingot-making and is formed as cap-state having height 9 separately covering the upper surface of the exothermic thermal-insulator 7 expanded in the feeder head part 4 in the feeder head brick 3 in the mold 1 for ingot-making, and composed of by mass% of 5-10% Al2O3, 75-80% SiO2 and the balance thermal-insulator composed of organic material, and mounted on the feeder head brick 3 in the mold 1 for ingot-making when a steel ingot is made.