Mathematical norms are important cultural knowledge of mathematical activities. This paper reports an analysis of mathematical norms in ten consecutive lessons taught by an eighth-grade Japanese teacher. The lessons were located in the unit of simultaneous linear equations. The videotapes of the lessons, their transcripts, and the interview data were analyzed qualitatively. Several major mathematical norms were found in the lessons. The teacher's deliberate strategies to develop them were identified: using students' work, making a comparison, and being considerate of those students who did not follow a norm. Complexities of research on mathematical norm are discussed. [For complete proceedings, see ED496851.]