A post-CMP washing liquid composition is provided which includes one type or two or more types of aliphatic polycarboxylic acids and one type or two or more types selected from the group consisting of glyoxylic acid, ascorbic acid, glucose, fructose, lactose, and mannose, and which has a pH of less than 3.0. This washing liquid has excellent performance in removing micro particles and metal impurities adhering to the surface of a semiconductor substrate after CMP and does not corrode a metal wiring material.