Sternoclavicular septic arthritis is a rare condition and it is usually related to predisposing conditions like intravenous drug abuse, diabetic mellitus, trauma and so on. A delayed diagnosis of this disease may cause severe complications like mediastinitis and chest wall abscess. Computed tomography or magnetic resonance imaging is needed to evaluate the complications. If the above complications are present, then joint resection should be considered. We report here on a case of a 52-year-old man who was diagnosed with primary sternoclavicular septic arthritis and he had no predisposing conditions. The pathogen on the aspiration-culture was S. aureus and it was susceptible to cefminox. The patient was cured with administering only antibiotic therapy for 6 weeks; intravenous cefminox therapy for 4 weeks followed by oral cefminox therapy for 2 weeks.