PURPOSE: To quickly detect the molten metal surface level in high precision in a twin roll type continuous caster.

CONSTITUTION: By watching what the height variation of molten metal surface is quickly changed according to the molten metal quantity, picture of the prescribed range to aim at boundary part 8 between a weir plate 3 and the molten metal 7 in the twin roll type continuous caster, is taken by a camera and pixel data in the range of a window 10a in the picture of a video taken by the camera are binarized into the molten metal part and the weir plate part and also, by obtaining the ratio of the pixel data in the molten metal part to the pixel data in the range of this window 10a, the height of molten metal surface level can be detected from variation of this ratio.