This research was aimed at describing the mathematical lateral thinking ability and self-confidence of students in Pythagoras Theory topic. This research applied the descriptive qualitative method. The subject of the research was a class of grade VIII E students of SMP Negeri 1 Ajibarang. The subject was chosen based on the final semester examination (UAS) score in the odd semester of the academic year 2017/2018. There were 3 students in each of the high-achiever, medium-achiever, and low-achiever groups. The data were analyzed through data reduction, data display, and verification. The data were collected through tests, questionnaires, interviews, and documentation. Based on the results of the research, it was concluded as follows. (1) Students in the low-achiever group had not been able to understand the lateral thinking ability questions and some of them
were able to deliver varied answers and create new ideas. The students were not confident yet and still compared themselves to others. Moreover, they were always bored when dealing with math problems they hardly found the answer. (2) Students in the medium-achiever group had been able to understand the lateral thinking ability questions. They were able to answer two questions for the same problem correctly and create new ideas from their own thought. However, they still compared themselves to others. (3) Students in the high-achiever group had
been able to understand the lateral thinking ability questions. They were able to answer two questions for the same problem correctly. They constructed the existed ideas to solve the problems, but they were not able to create new ideas from their own thought. They sometimes were bored solving math problems they hardly found the answer.