An inequality concept has an important role; even in advanced mathematics, inequality assists in analysis and proof. However, students' understanding of inequality is not satisfying. The fact shows that students experience difficulties and errors in solving inequality problems. These difficulties and errors are not intentional or do not result from students' carelessness in carrying out solutions or students' ignorance (misconceptions). Instead, these difficulties and errors are caused by epistemological obstacles. Therefore, this study explores the epistemological obstacles students face in the inequality concept by analyzing errors found in solving inequality problems. The qualitative research design with a phenomenological approach was chosen to achieve the research objectives by involving 29 eleventh-grade secondary students. The researchers employed a test on the inequality concept to explore students' epistemological obstacles, which consisted of three problems. A one-to-one unstructured interview was also conducted to investigate students' ways of thinking and understanding based on their answers. Furthermore, the data were analyzed using an inductive approach, combining systematic data management methods through reduction, organization, and connection. The data obtained are then presented in the form of narratives and figures. The results showed that the inequality rules, the absence of semantic and symbolic meanings of inequalities, interpreting solutions, and generalizations in the inequality rules become sources of students' errors in solving inequality problems. Thus, we found epistemological obstacles in the inequality concept based on these four types of errors. The obstacles are indicated by students' limitations in understanding and interpreting inequality signs as they solve inequality problems.