Aim: To evaluate the utility of plasma circulating tumor DNA (ctDNA) using the peptide nucleic acid-locked nucleic acid (PNA-LNA) clamp method to detect epidermal growth factor receptor (EGFR) mutations in non-small cell lung cancer (NSCLC) patients who had progressed under treatment with EGFR-tyrosine kinase inhibitors (TKIs). Patients and Methods: Blood samples were collected from patients with EGFR mutation-positive NSCLC who had progressed on EGFR-TKIs between March 2016 and August 2016 at the Kyoto University Hospital. Extracted ctDNA was analyzed using the PNA-LNA clamp method. In eligible patients, tissue re-biopsy was also performed and EGFR mutation status was compared between tissue and plasma samples. Results: Thirty-one patients were enrolled in this study. Known activating EGFR mutations and the T790M mutation were detected in 18 (58%) and 5 patients (16%), respectively. Twenty-five patients underwent tissue re-biopsy. Adequate samples for mutation analysis were obtained from 21 patients and 10 patients were found to be tissue T790M-positive. Among these 10 patients, 4 patients were positive for T790M in plasma ctDNA (sensitivity 40% and specificity 100%). All patients with T790M-positive plasma ctDNA responded to osimertinib. Conclusion: Sensitivity of the PNA-LNA clamp method in detecting the plasma EGFR T790M mutation was moderate with elevated, however, specificity. Plasma EGFR T790M testing may be adequate for the initial step; however, tissue re-biopsy should be considered for plasma T790M-negative patients because of its high false-negative rate.