<p>This study reveals the properties of inquiries within textbooks and identifies gaps between the new inquiry-based curriculum and textbooks, with the aim of enhancing textbook quality through the integration of different subjects and mathematical perspectives. With a conceptual framework that includes the degree of integration and level of inquiry, it focuses on two inquiry-based high school textbooks in Japan, employing a mixed method approach with both horizontal and vertical analyses. The analyses, both quantitative and qualitative, revealed that despite the curriculum guidelines’ intention to integrate various subjects, the degree of integration was minimal in nearly all targeted inquiries and was often limited to a disciplinary approach. These findings highlight significant gaps between the national curriculum standards and inquiry-based high school textbooks, particularly in terms of the degree of integration and levels of inquiry. The results also showed that the cross-curricular perspective is not adequately reflected, as many inquiries use mathematics and statistics merely as tools. Furthermore, the mathematical scope of these inquiries was limited, meaning the inquiries did not reflect the ideal cross-curricular inquiry-based education outlined in the curriculum. The paper argues that further consideration should be given to reconceptualising inquiry-based mathematics education from the cross-curricular perspective. In addition, it suggests that textbooks be further developed to feature inquiries that better harmonise with lesson studies created collaboratively by multi-disciplinary teachers and researchers involved in textbook development in the Japanese context. This Asian fine connection between curriculum and textbook development and practices could contribute to the development of cross-curricular inquiries.</p>