To improve the yield and operatability by providing a heat insulating seat plate having a smaller opening part than the opening area of an ingot mold and arranged between the bottom of an upper head frame and the upper surface of the ingot mold.

The heat insulating seat plate 6 is an integrally formed material having the opening part 6a smaller than the opening area of the ingot mold 2 and almost the same area as the opening part 4a of the upper head frame 4. Therefore, at the time of setting the mold head frame 1 to the upper end of the ingot mold 2, firstly, the light heat insulating seat plate 6 is put at the top of the ingot mold 2 and successively, the upper head frame 4 held with an upper head holding body 5 can be put on the heat insulating seat plate 6, and the setting work is easily executed.