[en] On 1996, Tono Earth Science Center of JNC introduced the mass spectrometer tandem accelerator for isotope analysis of natural samples such as underground water and rocks. Facilities were 15SDH-2 pelletron type system made by NEC (Nihon Electric Company), Japan. Total beam generation times were 949 hours from April 1999 to January 2000. We made measurements on total 795 samples from November 1999 to April 2000 using the ion source for solid samples. Samples were IAEA calibration standards, oxalic acid of NIST (National Institute of Standards), CaCO₃ from shells, BaCO₃ and SrCO₂ from underground water, CO₂ from air, and organic samples from soils. Preliminary tests using the ion source for gaseous samples have been carried out. We have used NIST oxalic acid as a standard and an IAEA C1 sample as background. We have got the lower values in the case of IAEA C2, C3 and C7 samples. Accumulation of the data is needed to confirm accuracy. In addition, we are accepting samples from outside after September 1999. Major troubles are: breaks of pellet chain, sparks near terminals, and damages of rotating shaft. The troubles are already fixed. We are continuing operation to estimate accuracy in relation with careful initial treatment of samples. (Y. Tanaka)