We have developed a fully automated measurement system of thermal conductivity combining a helium refrigerator and a data acquisition, processing and control system (NEC-9800 series) with RS 232 C-GPIB lines. The thermal conductivity is measured by a conventional steady-state heat flow method in the range of 15-200K. It is found that a measurement of a NIST standard reference material ensures high accuracy of 3% for this apparatus. Furthermore, the measured thermal conductivity of high-T_c oxide superconductor (BPSCCO) agrees with the typical data reported by a number of groups, and shows a clear enhancement below T_c.