[en] Boronization effects have been systematically investigated using discharges with identical conditions in JT-60U. In ∼50 shots after boronization using 70 g of B₁₀D₁₄, the carbon content decreased due to suppression of the chemical sputtering of carbon tiles while the boron content increased. In contrast to the boron and carbon contents, the oxygen content gradually increased from ∼0.7% to ∼1.3% in ∼500 shots. Continual boronization using 10-20 g of B₁₀D₁₄ every ∼200 shots successfully kept the oxygen content less than ∼1.0%. However, the durability of boronization using 20 g of B₁₀D₁₄ is much shorter than that of boronization using 70 g after vacuum vessel ventilation. Due to the reduction of the carbon and oxygen contents, Z_eff in 50 shots after boronization lies in the lowest range of the database