To provide a molten iron pretreating method which can continuously measure the molten iron surface level as a process during operation even if the measuring time of the molten iron surface level in the previous process of the pre-treating operation and a refining operation is not arranged, and can precisely measure the molten iron surface level without needing troublesome maintenance work.

The molten iron pre-treating method blows a refining agent into the molten iron from an injection lance by using carrier gas. In such a case, a level meter 3 and a vibration meter 2 are arranged to the injection lance 1 and the point of time when the lower end of the injection lance 1 rushes into the molten iron, is detected from the variation of the vibration waveform obtd. from the vibration meter 2, and the dipping depth of the injection lance 1 is set by using the injection lance at the point of rushing time as the reference.