The present invention provided methods for treating lung cancers using therapeutic agents for lung cancers comprising erbB receptor inhibitors as active ingredients. Methods for examining the responsiveness of lung cancer patients to erbB receptor inhibitors by using blood amphiregulin (AREG) as an indicator were provided. Furthermore, methods for treating lung cancers in which responsiveness to an erbB receptor inhibitor is determined based on blood AREG concentration, and in which therapeutic agents are selectively administered to patients expected to be responsive, were also provided. Advanced therapeutic effects can be expected by administering an erbB receptor inhibitor to patients predicted to be responsive. The present invention contributes to the improvement of therapeutic effects of gefitinib (Product name: Iressa®) and such on lung cancers.