To provide a method for decarburizing molten steel by which C concentration can be reduced without requiring a large facility investment and without causing the increase of a proportional cost of slag-making material and refractory.

The inner part of a vessel for holding a ladle, is made to a reduced pressure atmosphere or a vacuum atmosphere. Variations of flowing quantities of bottom gas blown into the molten steel in the ladle from the gas plugs arranged in the ladle, are respectively corresponded to the maximum value Xmax and the minimum value Xmin in the one side of the gas plug and the minimum value Ymin and the maximum value Ymax in the other side of the gas plug. The flowing quantity ratios of the minimum values and the maximum values Xmin/Ymax and Ymin/Xmax are defined as 0.5 times to stir the molten steel.