To effectively suppress generation of scale in a system even when a scale generation component may be mixed in the makeup water in a cooling water circulation type cooling system.

The circulation water 2 circulated in the system is partially extracted as the test water 14, the seawater 16 capable of being mixed in sewage disposal water 7 used as the makeup water and including the scale generation component, is added to the test water 14, the relationship between the additive amount of seawater 16 and the concentration of scale generation component dissolved in the test water 14 in which the seawater is added, is acquired, and a scale generation risk in the system in which the circulation water 2 is actually circulated, is determined on the basis of the relationship. Thus the scale generation risk in the system can be acquired while considering the probability of mixing of the scale generation component to the sewage disposal water 7 caused by the mixing of the seawater to the sewage water, and the generation of scale in the system can be effectively suppressed even when such the sewage disposal water 7 is used as the makeup water.