Here, we report a case of gastric neuroendocrine carcinoma showing an interesting tumorigenic pathway. A 57-year-old Japanese woman presented with epigastric tenderness, and distal gastrectomy was performed. In the surgical specimen, histologically, the tumor tissue was composed of three subtypes of tumor components showing different histological architecture and cellular atypia, diagnosed as neuroendocrine tumor(NET) G2, NET G3, and neuroendocrine carcinoma(NEC) components. Immunohistochemically, the Ki-67-positive rates of NET G2, NET G3, and NEC components were 6.5%, 99.5% and 88.1%, respectively. Although allelic imbalance(AI) on chromosomes 1p, 3p, 8q, TP53, 18q and 22 q was commonly found in all components, AI of 4p was found in NET G3 and NEC components(but not in the NET G2 component). In contrast, AIs of 5q and 9p were found in only the NEC component. Thus, we showed the progression from NET G2 to NEC, via NET G3, within the same tumor. Here, we report a case of gastric neuroendocrine carcinoma showing an interesting tumorigenic pathway. A 57-year-old Japanese woman presented with epigastric tenderness, and distal gastrectomy was performed. In the surgical specimen, histologically, the tumor tissue was composed of three subtypes of tumor components showing different histological architecture and cellular atypia, diagnosed as neuroendocrine tumor (NET) G2, NET G3, and neuroendocrine carcinoma (NEC) components. Immunohistochemically, the Ki- 67-positive rates of NET G2, NET G3, and NEC components were allelic imbalance (AI) on chromosomes 1p, 3p, 8q, TP53, 18q and 22 q were found in all components, AI of 4p was found in NET G3 and NEC components In contrast, AIs of 5q and 9p were found in only the NEC component. Thus, we showed the progression from NET G2 to NEC, via NET G3, within the same tumor.