MAT-251 mass-spectrometer installed at Shizuoka University, donated by the Thermo-Quest Co. Ltd., was renovated for precise carbon and oxygen isotopic measurement of very small amount of carbon dioxide. We examined the fundamental properties of mass-spectrometer, such as isotopic shift of reservoir CO2 during capillary leak separation, pressure effect on isotopic ratio and the correction of data with international standard material. We obtained consistent results that match the precision and accuracy with that of MAT-250 at the same laboratory. An online carbonate extraction apparatus was installed to the Inlet system for carbonate minerals including dolomite, calcite and aragonite using oil bath circulation heating system, which can attain temperatures up to 100°C. We describe the operation manual for precise measurement of small amount of carbon dioxide, derived from carbonate minerals reacted with conc. phosphoric acid and organic materials or graphite combusted in quartz tube.