A 59 years-old male consulted our hospital for loss of appetite which was maintained more than half a year. Abdominal contrast enhanced CT showed intraperitoneal tumor whose diameter was up to 17 cm, whole body lymph node enlargement, and multiple tumors of the liver. Although the liver biopsy and the neck lymph node biopsy failed to lead confirmed pathological diagnosis, laparoscopic tumor biopsy revealed intraperitoneal tumor was pathologically adenocarcinoma. The primary origin was unknown but imaging inspection indicated the origin might be small intestine. The chemotherapy (FOLFOX) was started according to standard regimen for advanced colorectal cancer. Although the tumor size was temporarily decreased to the diameter of 12 cm, it turned to be progressive two months afterward. The regimen was thus switched to FOLFIRI but the progression was unstoppable, caused acute renal failure, and perished him four months after the FOLFOX had started. The bereaved family's consent was obtained and the autopsy was done 1 h after the death. Intraperitoneal tumor with a diameter of the maximum of 15 cm was located on the small intestine, infiltrated into the bladder and the rectum, disseminated to the peritoneum, and caused lymph node metastasis in retroperitoneum and distant metastasis in the liver, both lungs, and pancreas. Pathological diagnosis was adenocarcinoma originating from the small intestine. Cancer of the small intestine is very as rare as 1% or less of all the alimentary canal malignancy and this case with autopsy can provide precious clinical information for cancer of the small intestine. We hope to report this precious case with bibliographic consideration.