An incineration system was constructed which were composed of a combustion furnace (AP-150R), a cyclone dust collector, radioisotope trapping and measurement apparatus and a radioisotope storage room built in the first basement of the Radioisotope Center.<BR>Low level radioactive samples (LLRS) used for the combustion experiment were composed of combustible material or semi-combustible material containing 500 kBq of ^99mTcO₄ or 23.25 kBq of ¹³¹INa. The distribution of radioisotopes both in the inside and outside of combustion furnace were estimated. We measured radioactivity of a smoke duct gas in terminal exit of the exhaust port. In case of combustion of LLRS containing ^99mTcO₄ or ¹³¹INa, concentration of radioisotopes at the exhaust port showed less than legal concentration limit of these radioisotopes. In cases of combustion of LLRS containing ^99mTcO₄ or ¹³¹INa, decontamination factors of the incineration system were 120 and 1.1, respectively.