PURPOSE: To obtain a corrosion inhibitor for metals which is safe in environmental sanitation, is stable when added and used in cooling water, etc., and has excellent rust preventive power by compounding borate, tungstate and molybdate properly selectively.

CONSTITUTION: The titled inhibitor is a corrosion inhibitor for metals consisting of borate, and tungstate and/or molybdate. salts of alkali metals or alkaline earth metals are adequate for the borate and sodium salts are more preferable. Sodium salts are more preferably used for the tungstate and molybdate as well. Compounding ratios of about 20W50wt% borate, and about 80W50% tungstate and/or molybdate are suitable, and the inhibitor contg. both at about 1:2 mixing ratios between the former and the latter is more particularly preferable. When the inhibitor of the above-mentioned compsn. is added at about 3,000ppm, it provides an effect. The use of the inhibitor in conjunction with other kinds of rust preventives is equally well.