PURPOSE: To accurately monitor electrode length and melted steel level height by lowering a support arm from a reference height at the upper electrode support arm height using an electrode length measurement reference pedestal installed outside a furnace and an electrode seat detector mounted on a wire end of an electrode elevation apparatus.

CONSTITUTION: An output signal is received from an electrode height location detector 11 and electrode elevation location heights B, D, and F are evaluated. Then, it is detected that an upper electrode makes contact with a pedestal B when an electrode measurement pedestal 13 is used, and there is evaluated a movement amount Lb obtained by the detector 11 when a height location C of the pedestal 13 is detected. Then, a reference electrode length Lc is evaluated on the basis of a distance between the height location C of the pedestal 13 and a reference height location A of an upper electrode support arm 7, and it is detected that an upper electrode 5 makes contact with a melted steel surface 15 upon arcing control and hence a movement amount Ly is evaluated on the basis of a melted steel surface height level E. The height location E is estimated on the basis of the heights C and A, and a fraction of a change in a charging amount is converted into a height fraction to estimate a melted steel surface height Lx.