We examined the relationship between the breaking stress of tofu made with 0.25% MgCl₂ and protein content, the ratio of 11S protein to 7S protein (11S/7S ratio), calcium content, and phytic acid content using six soybean cultivars: Enrei, Sachiyutaka, Fukuyutaka, Tachinagaha, Hatayutaka, and Ayakogane. Among the four components, protein content, 11S/7S ratio and calcium content showed cultivar differences, while the phytic acid content showed little cultivar variation. No single component was found for explaining the breaking stress of tofu and its cultivar difference. Among the six cultivars, only Ayakogane showed a significant correlation between the protein content and the breaking stress of tofu (r = 0.72*), which might be due to high variation of the protein content within Ayakogane. For the 11S/7S ratio, each cultivar showed a positive correlation, however, correlation coefficient was 0.56 at most. Tachinagaha, showing the highest calcium content, showed the lowest coefficient of determination for the relationship between the phytic acid content and the breaking stress of tofu. In contrast, Sachiyutaka, showing the lowest calcium content, showed the highest coefficient of determination. These results implied that the calcium affects the relationship between the phytic acid content and the breaking stress of tofu, however, further study is required to clarify the mechanism of the effect of the phytic acid and relationship between phytic acid and calcium.