Benign acute myositis by influenza virus usually occurs in children during recovery from the respiratory illness. It has been mostly reported in Japan and Europe but there has been no report confirmed by virus isolation in Korea. A 4-year-old boy was admitted to the hospital who presented with influenza-like symptoms, painful calves, and difficulty walking. The patient recovered within 5 days without any serious sequelae. We confirmed influenza B virus by RT-PCR and viral culture from clinical specimen. We report a case with acute myositis due to influenza B infection by clinical and laboratory diagnosis.