A 54-year-old man visited our hospital with right incidentally-found renal tumor detected by ultrasonography. Computed tomography, magnetic resonance imaging and angiography showed a small tumor, 1.5 cm in size, at the upper portion and a simple cyst, 4 cm in size, at the lower pole of the right kidney. We enucleated the small tumor and aspirated the cyst with outer part resection of the cyst wall. Pathological findings of the tumor showed renal cell carcinoma, alveolar type, common type, clear cell subtype, G1, pT1, INF-alpha. Microscopic appearance of the excised cyst wall also revealed sheets of renal cell carcinoma inside the wall. Therefore, two weeks after the first operation, we performed right radical nephrectomy. The resected specimen had severe inflammation without any evidence of residual tumor. Eight months after the nephrectomy, no recurrence has occurred.