Pyogenic sacroiliitis is an extremely rare disease in the field of obstetrics and gynecology, and especially for the cases discovered and treated during pregnancy, only five cases can be found in literature. We experienced one case of the disease in which the patient needed urgent hospitalization due to dysbasia caused by fever and pain at the left hip at the 27th week of her pregnancy. The patient was a 31-year-old primipara presenting typical clinical symptoms of pyogenic sacroiliitis along with evidence of severe infection as represented by fever of 39.7 degrees C and CRP of 12.6 mg/dl. She showed a good response to meropenem (MEPM) at 1 g twice a day for 8 days and then at 0.5 g twice a day for 2 days, followed by faropenem (FRPM) at 200 mg three times a day for 12 days, which successfully improved her subjective and objective findings as well as her laboratory test values, resulting in a complete cure. The definitive diagnosis of the disease in the patient was made on the basis of MRI findings, but no pathogen was identified. The patient was found to have marginal placenta previa as a complication, but she had an uneventful trans-vaginal delivery at the 37th week of her pregnancy and left hospital after both she and her baby showed favorable post-delivery progress. The case reported here is the first case of pyogenic sacroiliitis that has ever been discovered and treated during pregnancy in Japan.