A 48-year-old man was referred to our hospital to investigate shadows in the upper left lung field 3 months after the onset of symptoms. After the cough and runny nose subsided, the patient developed erythema and swelling of the left anterior chest and had difficulty raising his left arm. Chest computed tomography showed a mass shadow in the left upper lobe extending to the left pectoralis major muscle, and pericardial and bilateral pleural effusions were also observed. Cultures of bronchial lavage fluid, sputum, and blood did not reveal any significant bacteria, but a skin biopsy of the left precordial lesion yielded a large amount of milky-white pus, and a bacterial culture of the pus revealed Actinomyces and Fusobacterium spp. The patient was able to raise his left upper extremity immediately following the purulent drainage. A diagnosis of pulmonary actinomycosis with mixed infection and chest wall invasion was made. The symptoms improved with sultamicillin administration. In this case, mixed infection with Actinomyces and Fusobacterium, indigenous bacteria in the oral cavity, is thought to have caused infiltration of the chest wall and pericardium. Therefore, pulmonary actinomycosis should be considered as one of the differential diseases when chest wall lesions are observed.