To determine the mass correction factor(MCF) which is a factor to convert the observed raw ratio to the absolute value, CIT Pb shelf standard of California Institute of Technology and SRM-981 Pb standard of US National Bureau of Standards were measured with a 05RB type, surface ionization solid source mass spectrometer (ion beam radius 30 cm, 90° sector, max. acceleration voltage 10 kV) of JEOL Co. Ltd. installed in Tokyo National Research Institute of Cultural Properties. Using an electron multiplier as ion beam collector, our measurement showed larger correction factor than the factor of root mass (√<m_1/m_2> hitherto used by some authors. Our MCFs were 1.01024 (±0.1 %), 1.00411 (±0.05%) and 1.01022 (±0.05 %) for the ratios of ^<206>Pb/^<204>Pb, ^<207>Pb/^<206>Pb and ^<208>Pb/^<206>Pb respectively. These factors were obtained by comparing observed raw values with the values by Catanzaro(1967) and Catanzaro et al. (1968) for the CIT Pb and SRM-981 respectively. The lead isotope ratios of four galena samples corrected with these factors were in excellent agreement with the reported values for these samples.
