The present invention relates to a composition for treatment or metastasis suppression of cancers which includes a p34 expression inhibitor or activity inhibitor as an active ingredient. According to the present invention, the p34 protein knock-down causes monoubiquitination of PTEN and accordingly nuclear localization of PTEN is induced, as a result, an Akt pathway which is related to survival, proliferation, invasive properties and metastatic properties of tumors is inhibited, and thus there is an effect of significantly reducing clonogenic potential and tumor forming potential of various cancer cells which simultaneously express PTEN and NEDD4-1. Consequently, the p34 gene expression inhibitor or p34 protein activity inhibitor according to the present invention can be effectively used as a treatment agent or a metastasis suppression agent for cancers.