To obtain a new supplemental coolant additive which can be used satisfactorily with hard water and does not cause corrosion of a metal member, and to provide a manufacturing and a using method therefor.

This supplementary coolant additive contains a 4-6C-dicarboxylic acid, preferably adipic acid; and optionally a 9-12C-aliphatic dicarboxylic acid, an aromatic monocarboxylic acid, an aromatic dicarboxylic acid, and salts thereof; salts such as a molybdate, a nitrate, a nitrite, and a silicate; and a buffering agent. The coolant additive is appropriately replenished to a coolant the weight of which has been decreased by leakage or evaporation, in a diesel engine, a gasoline engine, or other cooling systems.