The biologic activity of individual cancer cells is highly heterogeneous. Hypoxia, one of the prominent features of a tumor microenvironment, is thought to be causal in generating this cellular heterogeneity. In this study, we revealed that primary lung cancer cells harboring activating epidermal growth factor receptor (EGFR) mutations generally entered a dormant state when hypoxic. We found that heterodimer formation of the ERBB family receptor tyrosine kinases (RTKs), and their subsequent downstream signaling, was diminished under hypoxic conditions, although phosphorylation of the EGFR was retained. Dormant lung cancer cells were found to be resistant to EGFR tyrosine kinase inhibitor (TKI) treatment. In terms of mechanism, we found that a negative regulator of ERBB signaling, MIG6/ERRFI1/RALT/Gene33, was induced by hypoxia both in vitro and in vivo. MIG6 expression prevented heterodimer formation of ERBB family RTKs, and suppressed their downstream signaling. Knockdown of MIG6 enhanced tumor cell growth under hypoxic conditions, and promoted the phosphorylation of ERK and AKT via increased EGFR-HER3 binding. Critically, sensitivity to an EGFR-TKI, as well as to irradiation under hypoxic conditions, was increased in MIG6 knockdown cells. The expression of MIG6 was partly correlated with a pS6 negative zone in patient tumors. Analyses of tumor sections from 68 patients with activating EGFR mutations showed that patients with high MIG6 expression showed significantly shorter survival after EGFR-TKI treatment than other groups. Collectively, our data suggest that dormant cancer cells with a high MIG6 expression level might be one of the causes of EGFR-TKI resistance in EGFR mutant lung cancer cells.