PURPOSE: To suppress the formation of scale in an aq. system occurring in zinc of the galvanized steel pipe or the like in a recovery system for boiler condensate by incorporating the polymer which contains polyglycine, polyalanine, etc. and is expressed by a specific general formula.

CONSTITUTION: The polymer expressed by the formula (wherein M denotes any one of the group of a hydrogen atom, sodium atom, potassium atom, ammonium atom, and m denotes 1 or 2) is incorporated, whereby a scale preventing agent for an aq. system is prepd. The mol. wt. of said polymer is usually 500W 300,000, more preferably 10,000W40,000 and the embodiment thereof is polyglycine, polyaniline, etc. The amt. of the above-described scale preventing agent to be added is usually 0.1W1,000ppm, more preferably 10W30ppm, and the aq. system to be added with said agent is a boiler aq. system and the agent is adaptable to any other system as far as zinc exists.