We previously reported that oral ingestion of Lentinula edodes mycelia (L.E.M.) extract can inhibit the growth of a subcutaneously established melanoma in a T cell-dependent manner via mitigation of regulatory T cell (Treg)-mediated immunosuppression. In this study, we tested the antitumor effect and mechanism of oral ingestion of L.E.M. extract following inoculation of murine colon carcinoma colon-26 (C26) cells into the subserosal space of the cecum (i.c.) of syngeneic mice. In this model, the primary site of the immune response was gut-associated lymphoid tissue (GALT), which is known to be an immunological tolerance-inducing site for numerous dietary antigens. Oral ingestion of the L.E.M. extract suppressed the growth of i.c.-inoculated C26 cells in a T cell-dependent manner and restored the T cell response of the mesenteric lymph nodes and the spleen, not only to a tumor antigen-derived peptide, presented on H-2Ld molecules, but also to C26 cells. I.c. inoculation of C26 cells increased the potential of CD4+ T cells of the mesenteric lymph nodes to produce transforming growth factor (TGF)-β, but ingestion of the L.E.M. extract decreased the ability of both CD4+ and CD8+ T cells in the mesenteric lymph nodes to produce this immunosuppressive cytokine. Although ingestion of L.E.M. showed only a marginal effect on Tregs in this model, this treatment significantly reduced the plasma levels of TGF-β and IL-6, both of which were increased in the i.c. C26-inoculated mice. In summary, our results indicate that oral ingestion of L.E.M. extract can restore antitumor T cell responses of mice even when the primary antitumor immune response is elicited in GALT, and provide important implications for anticancer immunotherapy of human colon cancer.