There is disclosed a process for cleaning a harmful gas which comprises bringing a harmful gas containing at least one member selected from the group consisting of nitrogen fluorides, tungsten fluorides, silicon fluorides, hydrogen fluoride and fluorine, especially nitrogen trifluoride into contact with a cleaning agent comprising stannous oxide as an effective ingredient at a temperature of 200.degree. C. at the lowest. The above process makes it possible to clean the harmful gas in a high performance at a relatively low temperature without by-producing a harmful gas or a gas with a fear of causing environmental pollution.