We present two adult cases of pneumococcal severe arthritis one case with, and other case without pericarditis those occurred in one season. Both cases eventually recovered, and the isolated S. pneumoniae strains were serotype 12F and penicillin susceptible. The patients were not elderly and had no medical history including pneumonia and vaccination. Although S. pneumoniae is a relatively rare causative pathogen of arthritis and/or pericarditis recently, a greater number of cases may be expected as a result of serotype conversion due to the increased use of pneumococcal conjugate vaccines in Japan.