The use of digital textbooks in mathematics is increasing. However, the implementation of digital textbooks in schools is complicated. The objective of this paper is to understand how the introduction of a digital textbook affects the activity of teaching mathematics. An intervention was conducted where six mathematics teachers, with students in aged 7-15, used a digital textbook for six weeks. This paper applies cultural-historical activity theory to consider contradictions that arose in the activity. Four contradictions were revealed: first, teachers' need for coherence and linearity vs. the digital textbook with a wide range of content and nonlinear paths; second, teachers' need for transparent learning processes vs. the digital textbook as opaque; third, teachers' beliefs about appropriate pedagogy for learning mathematics vs. the pedagogy embedded in the digital textbook; and fourth, differentiated instruction for all students vs. digital textbook supporting individualization for every student.