Emphasis has been placed on the behavior of ruthenium (Ru) in the evaporation to dryness accident due to the loss of cooling functions (EDLCF) of high-level liquid waste in fuel reprocessing plants. It is because Ru would form volatile compounds such as ruthenium tetroxide (RuO$_{4}$) and could be released into the environment with other coexisting gasses such as nitric acid (HNO$_{3}$), water (H$_{2}$O). To contribute to the safety evaluation of this accident, we experimentally evaluated the decomposition and chemical change behavior of the gaseous RuO$_{4}$ (RuO$_{4}$(g)) under the various atmospheric conditions: temperature and composition of coexisting gasses. As a result, the behavior of the RuO$_{4}$(g) was diverse depending on the atmospheric conditions. In the experiments with the dry air or H$_{2}$O vapor, decomposition of RuO$_{4}$(g) was observed. In the experiment with the mixed gas which containing HNO$_{3}$, almost no decomposition of the RuO$_{4}$(g) was observed, and chemical form of the RuO$_{4}$(g) was retained.