A 55-year-old man treated with oral metformin for type 2 diabetes mellitus was admitted for nausea, and hematemesis. From 4 days earlier, he had a history of alcohol abuse. He was diagnosed as having lactic acidosis by findings of metabolic acidosis (anion gap 43.3 mEq/l, pH 6.850), elevated serum lactic acid (250 mg/dl) and pyruvic acid (2.68 mg/dl). He also had hepatic and renal dysfunction. The compound combination of metformin accumulation, alcohol abuse, liver and kidney dysfunction, and inhibition of lactic acid metabolism apparently caused this severe lactic acidosis, which was ameriorated by hemodialysis. Liver and kidney dysfunction improved after 15 days of treatment. He was discharged from the hospital without sequela. Because lactic acidosis may lead to acute multiple organ failure, rapid diagnosis and intensive treatment including hemodialysis are necessary.