Pulmonary actinomycosis reportedly forms 15% of all cases of actinomycosis, and pulmonary Actinomyces odontolyticus is particularly rare. A 60-year-old man with a hoarse voice was referred to our hospital. Lung squamous cell carcinoma was diagnosed at the clinical tumor-node-metastasis stage of cT2N2M0, and concurrent chemoradiotherapy was initiated. Further, a small cavity was also detected in the left upper lobe, but it was observed. During chemoradiotherapy, the small cavity lesion rapidly increased accompanying infiltration, and administration of short-term antibiotics did not improve the patient's condition. Bronchoscopy did not show any diagnostic results. Although a rapidly progressive malignant lesion could not be excluded and surgical management was considered, resection could not be performed because of the tight adhesion of the mass. Therefore, bronchoscopy was performed again, and the bronchial lavage culture showed a positive smear for the Actinomyces species. Further, using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), the bacteria was identified as A. odontolyticus. After long-term administration of amoxicillin, the lung cavity with infiltration gradually improved. To the best of our knowledge, there have been nine cases of pulmonary A. odontolyticus (excluding those with only empyema or pleural mass without lung lesions), which can occur in immunocompetent patients with persistent lung shadow. None of the cases showed drastic deterioration; therefore, the present case is the first to highlight that A. odontolyticus possibly produce drastically progressive lung cavity lesion. Further, repeated bronchoscopy and MALDI-TOF MS could help to diagnose pulmonary actinomycosis.