First, at least one of silanol and a siloxane compound is generated in a chlorosilane (S101). In the step, for example, an inert gas having a moisture concentration of 0.5 to 2.5 ppm is brought into contact with the chlorosilane to dissolve the moisture, and at least one of silanol and a siloxane compound is generated through a hydration reaction of a moiety of the chlorosilane. Next, a boron-containing compound contained in the chlorosilane is reacted with the silanol or the siloxane compound, thereby converting the boron-containing compound to a boron oxide (S102). Through the step (S102), the boron-containing compound being a low boiling point compound is converted to a boron oxide being a high boiling point compound, and therefore the difference in boiling point from the boiling point of chlorosilane becomes larger to make later separation easy.