PURPOSE: To obtain an underwater anticorrosive having superior function to prevent the corrosion of an iron-based member in flowing water by incorporating Al and a chelating agent made of a polyamine having specified chelating abilities to Al and trivalent Fe.

CONSTITUTION: This underwater anticorrosive contains Al and a chelating agent made of a polyamine (e.g. diethylenetriaminepentaacetic acid capable of forming a chelate compd. with Al in an alkali region of ≥pH7 and having higher chelating ability to trivalent Fe than that to trivalent Al. It is desirable that the chelating agent and Al are used by nearly equimolar amts. This anticorrosive continuously produces a very high anticorrosive effect even in an aq. system whose pH is liable to rise and does not. generate scale.