The present invention provides a method of refining low purity Si by a slag, in particular removing B, which suppresses wear of the reaction vessel due to the slag and produces high purity Si used for solar battery materials etc. at a low cost, comprising adding SiO.sub.2 and an alkali oxide or alkali carbonate as a slag material into molten Si to form a slag during which adding one or more types of materials among materials the same as the reaction vessel material used or ingredients included in the reaction vessel material into the slag so as to remove the impurities in the molten Si.