PURPOSE: To provide a scale preventing agent which suppresses effectively the formation of scale due to Zn2+ ion with the concn. lower than for the case in which an acrylic acid polymer or its water soluble salt and ≥1 kinds among phosphonic acid, α-aminocarboxylic acid or the water soluble salt thereof are used alone.

CONSTITUTION: An acrylic acid polymer (e.g.; acrylic acid homopolymer) or its water-soluble salt (e.g.; Na salt) (component A) is prepd. The mol.wt. of the polymer is usually 500W300,000, more particularly 1,000W50,000. On the other hand, ≥1 kinds among phosphoric acid (e.g.; aminotrimethyl phosphonic acid), α-aminocarboxylic acid or the water-soluble salt thereof (component B) are prepd. The component A and the component B are combined at a compounding ratio of 95:5wt%W5:95wt%, whereby an intended scale preventing agent is obtd. The formation of the scale due to Zn2+ is effectively suppressed if such scale preventing agent is added to object water such as boiler feed water.