The beliefs of teacher regarding their students' learning in mathematics are an essential factor due to their importance in developing a desirable mathematics classroom culture. The purpose of this study is to investigate the mutual relationship between the construction of sociomathematical norms in classroom mathematical culture and teachers' beliefs. We observed 13 sessions of a fourth grade elementary mathematics class for two periods totaling one year, and conducted interviews with the elementary teacher and students who participated in this study. The study results revealed that the mathematical belief of the elementary school teacher was reflected in the decision making for mathematics instruction, and greatly influenced the purpose, contents and methods of the mathematics class, which contributed to creating sociomathematical norms, or repeated patterns appearing between the teacher and students.