The aim of this study was to analyze characteristics of teachers`` knowledge about reductio ad absurdum. In order to achieve the aim, this study conducted didactical analysis about reductio ad absurdum through examining previous researches and developed a questionnaire with reference to the results of the analysis. The questionnaire was given to 34 high school teachers and qualitative methods were used to analyze the data obtained from the written responses by the participants. This study also elaborated the framework descriptors for interpreting the teachers`` responses in the light of the didactical analysis and the data was elucidated in terms of this framework. The specific features of teachers`` knowledge about reductio ad absurdum were categorized into five types as a result. This study raised several implications for teachers`` professional development for effective mathematics instruction related to reductio ad absurdum.