To efficiently melt a chromium-containing steel having ≤0.01% carbon content under condition of restraining oxidation of chrominum.

A command signal generating means 21 transmits a signal showing molten steel wt. to a calculating means 23. A steel component analyzing means 22 transmits the analyzed value of the carbon content in the molten steel before vacuum treatment to the calculating means 23. After starting the vacuum treatment, an exhaust gas analyzing means 20 transmits the analyzed values of CO and CO2 concns. in the exhaust gas to the calculating means 23. An exhaust gas flow meter 19 transmits the detected value of the exhaust gas flowing quantity to the calculating means 23. The calculating means 23 calculates the carbon content and decarburizing velocity in the molten steel corresponding to each output, and further, obtains a target value of oxygen supplying velocity based on the preset equation and this value is transmitted to a control means 24. An oxygen flowing meter 16 transmits the detected value of the oxygen flowing quantity to the control means 24. The control means 24 controls the opening degree of the flowing quantity adjusting value 17 so that the detected value of the oxygen flowing meter 16 coincides with the target value.