[docdb]The present invention is capable of reducing the gas cost in a vacuum degassing and decarbonizing process by using a CO2 gas as a reflux gas and an agitation gas without causing the stoppage of the decarbonization, or an increase in the carbon concentration, ad an increase in the addition amount of an alloy. The present invention relates to a method of manufacturing low carbon molten steel by a vacuum degassing and decarbonizing process by carrying out the steps of blowing a CO2 gas into a furnace from the degasification starting time to the time of attainment of a carbon concentration in molten steel of 50 ppm in a vacuum degassing and decarbonizing process for the molten steel, and thereafter using an Ar gas.