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IgG4-related disease (IgG4-RD) has been recognized as a distinct disease entity, and the occurrence of IgG4-RD in the breast, is extremely rare. In this report, we describe a possible case of IgG4-related mastitis. A 55-year-old Japanese female presented with a left breast tumor. Ultrasound examination demonstrated a hypoechoic tumor in the left upper outer quadrant and lymph node swelling. The tumor had become larger, and a right breast tumor was also noted. Accordingly, surgical resection of the left breast tumor was performed. After the surgery, parotid gland tumors and multiple lung nodules were noted, and the serum IgG4 level was elevated (144 mg/dL). A histopathological study demonstrated dense lymphoplasmacytic infiltration with lymphoid follicle formation, fibrosclerosis and obliterative phlebitis. Immunohistochemical analyses revealed substantial IgG4-positive plasma cell infiltration ; however, the IgG4/IgG-positive plasma cell ratio was 23.6%. Accordingly, this patient was considered to have a possible case of IgG4-related mastitis. Additional studies are needed to clarify the clinicopathological features of IgG4-related mastitis because the diagnostic criteria for this disease have not yet been established., IgG4 関連疾患（IgG4-RD）は，一疾患として認識されており，乳腺におけるIgG4-RD は極めて珍しい．乳房におけるIgG4 関連疾患疑診群と診断した症例について報告する．症例は，55 歳の日本人女性で，左乳房腫瘤を自覚した．超音波検査では，左C 区域の低エコーと両側の腋窩リンパ節腫大を示した．その後左乳房腫瘤の増大と右乳房腫瘍が指摘されたため，左乳房腫瘤に対して腫瘤摘出術が行われた．術後，耳下腺腫瘍と複数の肺結節が出現し，血清IgG4 レベルが上昇していた（144mg/dL）．手術標本の病理結果は，リンパ濾胞周囲を主体に多数形質細胞が浸潤し，一部で線維化・硝子化を伴い静脈炎も観察された．免疫染色では，IgG4 陽性の形質細胞浸潤を認めたが，IgG4/IgG 陽性形質細胞比は23.6％であったため，IgG4 関連疾患疑診群の診断となった．この疾患の乳腺における診断基準がまだ確立されていないため，IgG4 関連乳腺炎の臨床病理学的特徴を明らかにするために症例の蓄積が必要である．