A 74 -year-old man was hospitalized for chest pain. Chest computed tomography showed a 4 × 8 cm-sized tumor pressing on the left pectoralis major muscle. Subsequent imaging showed progression of the tumor along with rib erosion and intrapleural invasion. The patient was admitted to our hospital for a follow-up examination of the tumor. Bronchoscopy results were normal. Ultrasound-guided percutaneous needle biopsy was performed. Histopathology indicated squamous cell carcinoma on the basis of the presence of keratin pearls. We therefore diagnosed the patient with squamous cell lung carcinoma fStage IIb (T3N0M0) and subsequently administered chemoradiotherapy owing to the inoperable status of the lesion. We report a case of squamous cell lung carcinoma that was difficult to differentiate from squamous cell carcinoma of the breast, along with a review of the literature.