A 69-year-old woman underwent left lower lobectomy for pStage IA lung adenocarcinoma in the left lower lobe in 20XX-17. She was admitted to our hospital because of carcinoembryonic antigen elevation, gait disorder, and disorientation. Brain magnetic resonance imaging showed high signal along the bilateral cerebral cortex on fluid-attenuated inversion recovery and diffusion-weighted imaging. A slight contrast enhancement on the brain surface was observed in the left frontal lobe on contrast-enhanced T1-weighted imaging, suggesting leptomeningeal metastases. Atypical cells were found on cerebrospinal fluid examination. EGFR deletion 19 was also detected in the cerebrospinal fluid and surgical specimen, and from the above we diagnosed leptomeningeal metastases as a postoperative recurrence of lung adenocarcinoma. After treatment with osimertinib, neurological symptoms improved and carcinoembryonic antigen levels decreased. The magnetic resonance imaging abnormalities also improved. We report this case because we consider it to be an extremely rare instance of leptomeningeal metastases that occurred 17 years after surgery for lung adenocarcinoma.