The mass fractionation correction factor to measure lead isotope ratios was obtained for a surface ionization mass spectrometer, Sector type-J manufactured by VG Isotope, which was recently installed in the Tokyo National Research Institute of Cultural Properties, Tokyo, Japan. The best conditions to get steady isotope ratios was searched for. Using the best conditions, the lead isotope ratios of the standard sample, NBS-SRM 981, were measured fully automatically, and the mass fractionation factor was calculated to be 1.00123 ±0.00005. Lead isotope ratios of 13 bronze samples were obtained using this factor and the ratios were compared with the values obtained by the former mass spectrometer, JEOL-05 RB of the Institute. The values measured by both mass spectrometers agreed well within the error range, and it was confirmed that the values obtained by the new apparatus can be treated in the same manner as those obtained by the former apparatus without any correction factor.