PURPOSE: To produce a corrosion inhibitor in water system low in toxicity and exhibiting excellent corrosion inhibiting effect to iron based metallic materials by preparing the corrosion inhibitor composed mainly of phosphinic acid derivative expressed by a formula and molybdic acid as valuable components.

CONSTITUTION: The corrosion inhibitor for the iron based metallic material in water system containing the phosphinic acid derivative expressed by the formula (where, R is -COOX' or -CONH-C(CH3)2CH2SO4X' and two kinds of the groups is coexisted in the same molecule, Y is hydrogen, alkali metal atom, -SO3X" or -PHOOX", each of X, X' and X" is hydrogen or alkali metal atom, each of (m) and (n) is 0 or positive integer and m+n is integer 1-100) and molybdic acid or its salt is prepared. And the weight ratio of the phosphinic acid derivative to the corrosion inhibitor is (1:10) to (10:1) and the content of the valuable components in water is controlled to 1-1000mg/l. As a result, the inhibitor free from the environmental contamination and capable of efficiently preventing the corrosion of the iron based metal at a low conc. is obtained.