The monitoring of impurity in sodium is one of the most important R&D issues to develop a liquid metal reactor. Impurity measuring experiments were carried out by a Plugging Temperature Indicator, which is an on-line impurity monitoring instrument. Plugging temperature was successfully measured at various oxygen contents with two operation modes- bare orifice mode and partially plugging mode. The relation between plugging temperature and oxygen content was calibrated from the experimental data. Further study is needed to develop a more reliable instrument.