ApcMin/+ mouse, a mouse model for human familial adenomatosis polyposis, contains a truncating mutation in the Apc gene and spontaneously develops intestinal tumors. Our previous study revealed two distinct stages of tumorigenesis in the colon of ApcMin/+ mouse: microadenomas and macroscopic tumors. Microadenomas already have lost their remaining allele of the Apc and all microadenomas show accumulation of β-catenin, indicating that activation of the canonical Wnt pathway is an initiating event in the tumorigenesis. This study shows that expression of nuclear β-catenin in macroscopic tumors is further upregulated in comparison with that in microadenomas. Furthermore, transcriptional activity of β-catenin/T-cell factor (Tcf) signaling, assessed using β-catenin/Tcf reporter transgenic mice, is higher in the macroscopic tumors than that in microadenomas. In addition, the expression level of Dickkopf-1, which is known to be a negative modifier of the canonical Wnt pathway, was reduced only in colon tumors. These results suggest that activation of β-catenin/Tcf transcription plays a role not only in the initiation stage but also in the promotion stage of colon carcinogenesis in ApcMin/+ mice.