This research aims to test the validity and reliability of the non-test instrument on the Mathematical Thinking Belief questionnaire, which was tested on class XI students. The method used in this research is quantitative survey research. The survey was given to 66 students in grades XI and X. The data collection technique was done by boldly filling out a questionnaire using a Google Form. The research instrument was developed as a self-reported questionnaire. The questionnaire consisted of 25 items on a 5-point Likert scale. This study uses the Rasch model using Winstep to analyze the data. The Rasch model is also used to determine the validity and reliability of the data in the form of a Mathematical Thinking Belief questionnaire. In the validity test, it can be seen from the MNSQ, ZSTD, and Pt Mean Corr values. Data analysis using Winstep shows that 24 items have good quality. The responses given by students are of good quality. So the analysis results using Winstep demonstrate that the Mathematical Thinking Belief instrument can be said to be reliable. The validity test using Ms. Excel is said to be invalid. reliability test using Ms.Excel is included in the very high category.