PURPOSE: To manufacture ferroboron at a low cost by using refining slag contg. boric acid produced as a by-product in a stage for manufacturing low-Al ferroboron as part of a starting material contg. boron used together with a starting material contg. iron so as to reutilize boron in a high yield.

CONSTITUTION: The starting material contg. boron and a starting material contg. iron is brought into a thermit reaction in the presence of Al powder to form high-Al ferroboron. This high-Al ferroboron is melted by heating, and Al in the molten ferroboron is oxidized by adding molten boron oxide to manufacture low- Al ferroboron. In this stage, refining slag contg. boric acid is produced as a by- product. The refining slag is used again as at least part of said starting material contg. boron for high-Al ferroboron. By this method, the yield of boron is increased, and low-C and low-Al ferroboron can be manufactured advantageously.