Non-lactational mastitis (NLM) consists of a broad spectrum of inflammatory breast diseases. Among them, periductal mastitis and idiopathic granulomatous mastitis(IGM) are the most commonly encountered in clinical practice. IGM can be diagnosed when all possible causes of granulomatous diseases (known mycobacterial or fungal infection, Wegener granulomatosis, sarcoidosis, etc.) are excluded. There is no consensus on the standard treatment of IGM patients. Surgery, oral corticosteroids, topical steroids, immunosuppressive therapy, and observation alone were all proposed as the treatment for IGM patients. However, all of the evidences are derived from retrospective studies. In a previous study, the investigators conducted a single-arm, proof-of-concept trial that showed the safety and efficacy of ductal lavage as a treatment for IGM patients. Ductal lavage is possible to spare the patients from corticosteroids therapy and or surgery. To further address this issue, the investigators are going to conduct this multicenter, randomized, open-labeled, non-inferior trial, to compare the efficacy and safety of ductal lavage and corticosteroids therapy. The aim of this study is to provide evidence for the management of IGM patients.