To efficiently obtain the highly pure metal silicon having a boron concentration reduced to a specific value or less at a low cost by blowing the mixture gas of steam, hydrogen and an inert gas into melted silicon to oxidize boron contained as an impure substance and then removing the produced slag.

Highly pure metal silicon having a boron concentration of ≤3 ppm is obtained. When a mixture gas obtained by adding steam and hydrogen to an inert gas is used as an oxidizing gas, such a gas composition that can properly control the oxidizing property of the gas and can gradually produce silicon dioxide without immediately producing the coating film of the silicon dioxide on the bubbles of the blown gas is obtained. In the case of the conditions, the boron can be brought into contact with the oxidizing gas and then removed as slag. Silicon 2 melted in an arc reducing oven is transferred from a pouring port 1 to a ladle 3, and Ar, H2O and H2 are mixed and blown from gas cylinders and a boiler through pumps 5, pipes 6 and a porous plug disposed in the bottom of the ladle 3 into the melted silicon 2 in the ladle 3.