In the present study, the anti‐tumor mechanism of Z‐100 was investigated with the use of pulmonary metastasis of B16F10 melanoma. In B16F10 mice, Th1 cytokine production (IL‐2, IFN‐γ) was suppressed in comparison with normal mice. On the other hand, Th2 cytokine production (IL‐4, IL‐10) was increased in the B16F10 mice. The administration of Z‐100 to B16F10 mice restored the balance of Th1/Th2 cell responses from the Th2 dominant state to the normal state. Z‐100 significantly suppressed the pulmonary metastasis of B16F10 melanoma in a dose‐dependent manner. These results suggest that Z‐100 restored the breakdown of Th1 cell responses, resulting in the suppression of pulmonary metastasis of B16F10 melanoma. Moreover, Z‐100 decreased the corticosterone levels, which is known to suppress the Th1 cell responses, in both serum specimens and splenic tissue, and the steroidogenic CYP11A1 mRNA expression in CD4+ T cells. These results suggest that a suppression of pulmonary metastasis and restoration of Th1/Th2 cell responses by Z‐100 may be due to the decrease in the corticosterone levels and the steroidogenic CYP11A1 mRNA expression of CD4+ T cells in B16F10 mice. Further, the role of Th1 cytokine, IFN‐γ, on these activities of Z‐100 was examined. The suppressive effects of Z‐100 on pulmonary metastasis and restoration of Th1/Th2 cell responses were eliminated by the administration of anti‐IFN‐γ mAb. Moreover, the suppressive effects of Z‐100 on glucocorticoid‐genesis were eliminated by the administration of anti‐IFN‐γ mAb. These results suggest that Z‐100 restores the balance of Th1/Th2 cell responses via the suppression of glucocorticoid‐genesis by Z‐100‐induced IFN‐γ. IFN‐γ acts as a key cytokine in anti‐tumor activities of Z‐100.