A 62-year-old woman presented to a primary care doctor on January 2006 due to a sore throat and high fever, and had received medication for a common cold. She was referred to our hospital in February 2006 because of additional manifestations such as painful rashes on the lower limb similar to erythema nodosum and polyarthralgia on her feet, shoulder and finger joints. She was initially treated with an anti-inflammatory drug (NSAID) for polyarthritis but the symptoms did not improved. In addition, the serum level of anti-streptolysin O antibody (ASO) was elevated at the second visit more than that at the first visit. She was diagnosed to have rheumatic fever (RF) based on the polyarthritis, inflammatory data and an increase of the ASO level. She was treated with 10 mg a day of prednisolone (PSL) and sultamicillin tosilate. However, a systolic murmur that had been never noticed by previous auscultation appeared after the third hospital day and the mitral regurgitation was also detected on echocardiogram. She was then treated with 40 mg a day of PSL because of an appearance of the carditis due to RF. The increased PSL dose promptly improved the systolic murmur as well as the arthritis. This report presented an RF case with carditis detected by an development of the systolic murmur in an adult female.