A 79-year-old man consulted us because of left chest pain and fever ranging from 38.0 to 38.9 °C. A chest computed tomography scan showed a mass lesion (φ40 mm) in the left lingular segment, and inflammatory markers were elevated. He was admitted with a diagnosis of lung abscess, and panipenem/betamipron was administered at a dose of 2 g/day, after which the symptoms showed slight transient resolution. However, his body temperature increased again, to more than 39.0 °C, on the eighth day of hospitalization, and a chest radiograph suggested pleuritis as a complication. The antibiotics were changed to teicoplanin (TEIC; 400 mg/day) and meropenem (2.0 g/day). Thoracic drainage and pleural lavage were initiated at the same time. Lactobacillus spp. was detected from the pleural effusion by culture and was considered to be the causative organism, and it was resistant to TEIC. Therefore, the antibiotic was changed, to clindamycin, to which the bacteria was susceptible. No subsequent fever or pleural fluid retention was observed. The patient’s course was good, and he was discharged on day 45 of hospitalization. Subsequently, the causative organism was identified as Lactobacillus rhamnosus by the 16s rRNA sequence. Lactobacillus rhamnosus is rarely pathogenic in humans. Lactobacillus rhamnosus infection mostly occurs in immunosuppressed patients, and only a few cases have been reported in immunocompetent patients. In the present case, the patient was not immunodeficient; however, his lung had an impaired local immunosystem, due to emphysema.