PURPOSE: To enable a molten metal level height detectable in high precision by converting a pressure variation to a height variation after a displacement of molten metal level is converted to a gas pressure variation and converting it to a molten metal level variation.

CONSTITUTION: An air of the prescribed pressure is sent as shown by the arrow marks 12, 13 from the airpipes 5, 6. The mercury level connected to the branch pipe 7 of the U shaped pipe 9 and the other end open to air shows the height difference H1 corresponding to the immersed depth H of the detection pipe 3. Accordingly, the molten metal level height is detected by converting H1 to the molten metal level height. When a molten metal level reaches the lower end of the detection pipe 3, the water height difference in the U shaped tube becomes zero and the molten metal level is lower than the upper limit. Next, when the lower end of the detection pipe 4 is at H depth from the molten metal level, the height difference of the mercury level of the U shaped pipe 10 corresponding to H0 is H2 and H2 becomes smaller by the molten metal level variation. When a molten metal level reaches the lower end of the detection pipe 4, the height difference of the mercury level of the U shaped pipe becomes zero. By this method, the upper-lower limit of molten metal are precisely measured.