We experienced a case of pyomyositis caused by Streptococcus pyogenes in the paravertebral muscles. A 41 year old man was admitted for the treatment of severe low back pain and fever. Magnetic resonance imaging (MRI) revealed inflammation in the right paravertebral muscle. S. pyogenes was cultured from the blood sample. He was diagnosed with pyomyositis and treated with antibiotics. Pyomyositis can become severe; thus it is important to make an accurate diagnosis and initiate treatment immediately. MRI is helpful for early diagnosis of pyomyositis in paravertebral muscle.