A 74-year-old woman underwent colonoscopy for positive fecal occult blood test. A colonoscopy revealed a Type 1 tumor in the rectosigmoid region. The tumor was diagnosed as well-differentiated adenocarcinoma(tub1)by biopsy. Laparoscopic high anterior resection was performed. The final diagnosis was MiNEN(adenocarcinoma:NEC=6:4), RS, pT4a(SE), INF c, Ly1c, V1b, Pn1b, BD2, pN2a(5/28), cM0, pStage Ⅲc. All lymph node metastases were of NEC origin. This case was considered to be at high risk of recurrence and require adjuvant chemotherapy focused on NEC. She was referred to an advanced medical institution for carboplatin and etoposide therapy. MiNEN is a rare disease, and has a poor prognosis. In order to establish a therapeutic strategy of MiNEN, it is important to accumulate further cases and evidence.