PURPOSE: To accurately measure slug thickness by measuring the electrical change at the slug upper surface and molten metal upper surface through a preparation electrode and a measuring electrode in the process of lowering and raising the measuring electrodes.

CONSTITUTION: Preparation electrode 5 is connected to molten metal 11, for example, it is inserted inside from the openings above furnaces 10 and the tip end is allowed to reach the molten metal 11. Then, measuring electrode 6 is inserted in the furnace 10. The voltage change in both electrodes 5, 6 are confirmed by the indication value of voltage change detection means 3, and the measuring electrode is gradually lowered from the position above the molten slug 12 so as to search a first change point showing a large voltage change between the electrodes 5 and 6. This point is recognized as the reaching point of the measuring electrode 6 to the molten slug 12 upper surface. Then the measuring electrode 6 is lowered further so as to search a second change point showing no voltage change between the electrodes 5 and 6. This point is regarded as reaching point of the measuring electrode 6 to the molten metal 11 upper surface.