A 77-year-old man was given a diagnosis of pT4aN0M1a(PUL2), stage Ⅳ, RAS mutant type, after the operation for advanced ascending colon cancer. He was administered mFOLFOX6 plus Bmab as first-line chemotherapy. He showed consciousness disturbance on the 2nd day during the 6 cycles. Because of head computed tomography and magnetic resonance imaging showing no abnormal findings, we diagnosed convulsive seizure. His consciousness level gradually improved after intravenous infusion. He showed consciousness disturbance on the 2nd day during the 7 cycles again. Because blood ammonia level were high at 400μg/dL, he was diagnosed as hyperammonemic encephalopathy. His consciousness level rapidly recovered after branched chain amino acid(BCAA)infusion. SOX plus Bmab therapy was started as a post-treatment, he developed hyperammonemia(NH3 288μg/dL)again, on the 4th day during the 3 cycles. After taking of oral administration of BCAA and lactulose, the recurrence of hyperammonemic encephalopathy was not found. Therefore, 3 cycles of SOX plus Bmab therapy and 12 cycles of IRIS plus Bmab therapy were administered.