We describe a case in which erlotinib treatment contributed to amelioration of malignant peritonitis as a result of lung adenocarcinoma after gefitinib failure. A 57-year-old man underwent right upper lobe resection, lymph node dissection, and partial lower lobe resection of multiple primary cancer for lung adenocarcinoma (pT1N0M1 stage IV). The operative sample was positive for the EGFR mutation (exon 18 G719X), and right malignant pleural effusion increased one year later. After gefitinib treatment, the pleural effusion was decreased, but three months later, it again increased. Thus we diagnosed his condition as gefitinib failure. We attempted various antineoplastic drugs, but he ultimately developed malignant peritonitis, and his condition deteriorated. We treated him with erlotinib; the ascites then decreased and the patient's condition improved, although liver metastasis was shown. But we experienced a case in which erlotinib treatment contributed to amelioration of malignant peritonitis as a result of lung adenocarcinoma after gefitinib failure.