|| Analytical method for strontium isotope ratios and rubidium and strontium concentrations has been established using a full automatic thermal ionization mass spectrometer. The machine is a modified model "MAT 260" of Varian MAT LTD. Each of simultaneously loaded thirteen samples is successively analysed full automatically following to a specific controling program which has been selected as being most suitable for each sample. However, the most characteristic feature of this machine compared to other types is computer controled peak jumping and peak centering before measurment of the signal of each peak. By this the accuracy of isotopic measurement has been surprisingly improved. Analytical procedures are described in detail which include decomposition of samples, separation of Rb and Sr, loading samples on filament and mass spectrometry. Accuracy and reproducibility of isotope analyses are excellent : 43 separate analyses of standard sample NBS 987 over one year gave a mean (87)Sr/(86)Sr ratio of 0.710238 (normalized to the (88)Sr/(86)Sr ratio of 8.375209) with a value of 20δ of 0.000008. The ratio obtained is slightly higher than the value of 0.71014 given by NBS, but it is almost identical to the mean of recently reported twenty analyses. Our determinations for Rb and Sr concentrations of standard samples are as follows : JG-1, Rb 184.4 ppm, Sr 185.5 ppm; JB-l, Rb 41.5Ppm, Sr 448.4 ppm , each of which is near the mean of reported values for corresponding element of the sample (ANDO et al., 1974). Concentrations of Rb and Sr in pure water and regents used in chemical treatement of samples were also measured; a possible error due to the contamination is negligible for most geochemical samples.