Precise value of strontium isotope ratio (⁸⁷Sr/⁸⁶Sr) can be obtained with a newly installed automatic mass spectrometer. The VG Sector-J, equipped with multicollector system, affords the measurement of ⁸⁷Sr/⁸⁶Sr ratios of 10 samples in a run within 30 hours with the use of about 400 ng of strontium for a sample. The ⁸⁷Sr/⁸⁶Sr ratio of NBS SRM 987 isotope standard reference sample was obtained to be 0.710244 + 0.000007 as an average value of four measurements. The ⁸⁷Sr/⁸⁶Sr ratios for eight standard rock samples prepared by the Geological Survey of Japan were determined after strontium was purified by the usual column method. Values obtained for the standard rocks were 0. 71096 (JG-1), 0.70416 (JR-1), 0.75546 (JR-2), 0.70358 (JA-1), 0.70412 (JB-1), 0.70364 (JB-2), 0.70343 (JB-3) and 0.70519 (JGb-1). These agree with the values previously published by different authors. It is shown that the method used in this report can be applied for the determination of ⁸⁷Sr/⁸⁶Sr ratios of earthenwares or stone implements of archaeological interest.