The vasohibin (VASH) family consists of two genes, VASH1 and VASH2. VASH1 is mainly expressed in vascular endothelial cells and suppresses angiogenesis in an autocrine manner, whereas VASH2 is mainly expressed in cancer cells and exhibits pro-angiogenic activity. Employing adenomatous polyposis coli gene mutant mice, we recently reported on the role of Vash2 in the spontaneous formation of intestinal tumors. In this study, we used K19-Wnt1/C2mE (Gan) mice and examined the role of Vash2 in spontaneous gastric cancer formation. Gan mice spontaneously develop gastric tumors by activation of Wnt and prostaglandin E2 signaling pathways in gastric mucosa after 30 weeks of age. Expression of Vash2 mRNA was significantly increased in gastric tumor tissues compared with normal stomach tissues. When Gan mice were crossed with the Vash2-deficient (Vash2LacZ/LacZ ) strain, gastric cancer formation was significantly suppressed in Vash2LacZ/LacZ Gan mice. Normal composition of gastric mucosa was partially maintained in Vash2LacZ/LacZ Gan mice. Knockout of Vash2 caused minimal reduction of tumor angiogenesis but a significant decrease in cancer-associated fibroblasts (CAF) in tumor stroma. DNA microarray analysis and real-time RT-PCR showed that mRNA levels of epiregulin (Ereg) and interleukin-11 (Il11) were significantly downregulated in gastric tumors of Vash2LacZ/LacZ Gan mice. Furthermore, conditioned medium of gastric cancer cells stimulated migration of and α-smooth muscle actin expression in fibroblasts, whereas conditioned medium of VASH2 knockdown cells attenuated these effects in vitro. These results suggest that VASH2 plays an important role in gastric tumor progression via the accumulation of CAF accompanying upregulation of EREG and IL-11 expression.