This study aims to analyze the test results of junior high school students' errors in solving mathematical understanding problems on set material in terms of Nolting's theory, which is a theory that divides into 6 types of errors: misread-directions errors (errors in reading instructions), careless errors (carelessness), concept errors (concept errors), application errors (application errors), test-taking errors (test-taking errors), and study errors (errors in the learning process). The method used in this method is descriptive qualitative with the research subjects of class VIII students at SMP Negeri 1 Parongpong. Data collection was carried out by giving tests as many as 4 description questions and processed using Microsoft Excel.