A 72-year-old man was referred to our urology department due to a giant adrenal tumor detected by computed tomography( CT). Endocrine screening showed that cortisol, renin, aldosterone, adrenaline, and noradrenaline levels were all normal, and there was no evidence of adrenal hyperfunction. The adrenal tumor was so large that we suspected malignancy. Contrast-enhanced CT of the abdomen was performed for qualitative diagnostic purposes, and showed wall thickening of the sigmoid colon extending for approximately 6 cm. Lower gastrointestinal endoscopy was performed and revealed a full circumferential type 2 tumor in the sigmoid colon. Biopsy results showed intermediate differentiated ductal adenocarcinoma. Tumor markers were as follows: CEA 23.1 ng/mL, CA19-9 962 U/mL. The adrenal tumor was suspected of being malignant due to its size, but imaging examinations did not lead to a diagnosis of primary or metastatic disease. There were no tumors other than those in the sigmoid colon and adrenal glands. Since complete resection was deemed possible, sigmoid colon resection and combined left adrenalectomy were performed for both a diagnosis and treatment. A histopathological examination revealed that the histology of the adrenal tumor resembled that of colorectal cancer, leading to a diagnosis of left adrenal metastasis from sigmoid colon cancer.