PROBLEM TO BE SOLVED: To provide a method for adding magnesium with high vapor pressure to molten steel and a device therefor.SOLUTION: A device for adding Mg granule to molten steel comprises a storage tank 11, a control valve 12 and a body 13. The body 13 has a transport pipe 131, a fitting pipe 132 and three way pipe 133. In a method for adding magnesium with high vapor pressure to molten steel, a device 1 for adding the Mg granule is delivered to a tubular furnace 2, pure Mg granule with particle diameter of 0.5 to 2 mm is filled into the storage tank 11 and a temperature of a transport pipe under construction part is increased to 1120 to 1200°C, then the control valve 12 is opened and the pure Mg granule is added to transport pipe pairs at a rate of 0.5 to 1.5 g/min. At the same time, Ar gas adjustment valve is opened and Ar gas is inflowed at a rate of 0.5 to 1.5 g/min, and Mg vapor is added to molten steel in the transport pipe 131.EFFECT: The present invention has advantages such as capable of stably
