Idiopathic granulomatous mastitis is an inflammatory disease with unknown etiology. The clinical signs and mammograms of the disease can mimic breast cancer. The present study reports a case of idiopathic granulomatous mastitis in a pregnant woman. A 26-year-old woman, who was 24 weeks pregnant, felt a mass on her left breast. Biopsy of the mass revealed no evidence of malignancy. For treatment, intravenous and oral prednisolone tablets were prescribed during the pregnancy, which was terminated without a problem under normal care.