[en] There are alkalifusion and pyrohydrolysis methods as a pretreatment for the mass analysis of boron isotopes in B₄C being used for control rods. In this study pyrohydrolysis method was selected, in which H₃BO₃ is recovered in distilled water instead of the weak alkaline solution and then the distilled water is evaporated to dryness yield crystal H₃BO₃. A sample was made for the mass analysis, adding an equivalent of Na₂CO₃ to crystal H₃BO₃ and dissolving it with a small quantity of distilled water. In an improvement of a reactor tube of quartz for pyrohydrolysis, a platinum cylinder was placed in the reactor tube to prevent the inner wall of the quartz tube from being corroded by the scattered sample and the adaptor was installed on the recovery side of H₃BO₃ so as to facilitate the recovery of the H₃BO₃ crystal precipitated. Through these improvements, inter contamination of samples due to the corrosion of the inner wall of the reactor tube was eliminated and the disturbance due to excess alkali almost disappeared with the addition of an equivalent of Na₂CO₃ to H₃BO₃ recovered, which made it possible to obtain a stable mass spectrum. Since the analysis can be made with the same conditions as the analysis of the standard sample (H₃BO₃) with a known isotopic composition, the cause of the occurrence of bias in analytical values due to the change in mass discrimination factor was lessened. Owing to the above-mentioned improvement, the repeatability of mass spectrum was reduced from over 1% to 0.2 -- 0.5%. (auth.)