The expression of the Wnt-antagonist Dickkopf gene (DKK) is downregulated in several types of tumors as a consequence of epigenetic DNA modification; four DKK members, DKK1, DKK2, DKK3, and DKK4, have been identified. In this study, we investigated another function of DKK3 in non-small cell lung cancer H460 cells, in which DKK3 was hypermethylated (44%) but still expressed, by interfering with DKK3 expression using DKK3-silencing RNA (SiRNA). We found that knockdown of DKK3 expression by DKK3 SiRNA transfection led to the detachment of H460 cells from the bottom of the culture plate and caused apoptosis. The expression of cyclin-dependent kinases D1 and E were increased by DKK3 knockdown, indicating that cells with blocked DKK3 expression entered the apoptotic pathway. We also found that the intracellular level of reactive oxygen species was higher in cells with blocked DKK3 expression than in normal H460 cells, and levels of p53, p21, and Bax were also increased by the gene knockdown. These results indicate that DKK3 acts as an antiapoptotic molecule by decreasing the intracellular level of reactive oxygen species.