To provide a contamination condition monitoring apparatus for a circulation water system in which a contaminant in circulation water can be detected accurately on an early stage and the contaminant can also be detected easily and inexpensively.

A contamination condition monitoring apparatus comprises a cell 1 which flows in such a manner that circulation water W within a circulation water system 100 passes therethrough, and transparent granular substances 2 which are brought into contact with the circulation water W and fill the inside of a transparent part 12 of the cell 1 so as to deposit contaminants in the circulation water W thereon for monitoring the condition of depositing the contaminants as to indicate the condition of contamination of the circulation water system. The granular substances 2 to which the contaminants are deposited are not transparent and, if the quantity of contaminants in the circulation water W is much, the range of depositing the contaminants to the granular substances 2 is widened toward a flowing direction of the circulation water W. Therefore, the condition of depositing the contaminants to the granular substances 2 is observed through the transparent part 12 of the cell 1 filled with the granular substances 2, thereby accurately monitoring the condition of contamination of the circulation water system 100.