PURPOSE: To produce a one pack type water treating agent excellent in bactericidal and algaecidal action by incorporating glutardialdehyde, a phosphinic acid derivative, molybdic acid, a zinc salt and an azol as valuable components.

CONSTITUTION: A rust preventive containing 1-30wt.% glutardialdehyde, 1-20wt.% phosphinic acid derivative expressed by the formula (where, R is -COOH or -CONH-C(CH3)2CH2SO3X' and two kinds of the group are coexisting in the same molecule, Y is hydrogen or alkali metal atom or the like, each of X and X' is hydrogen or alkali metal atom, each of (m) and (n) is 0 or positive integer number and m+n is integer 1-100), 1-15wt.% molybdic acid or its derivative (sodium molybdate or the like), 1-10wt.% zinc salt (zinc chloride or the like) and 0.1-10wt.% azol (1,2,3-benzotriazol or the like) as valuable components is prepared. As a result, the one pack type water treating agent having corrosion and scale inhibiting action and having the resistance to the corrosion of iron base metallic materials is obtained.