Abstract: When the water stored in a reservoir 1 and containing chlorine ion and/or bromine ion is supplied to and circulated through an electrolyzer 4 and electrolyzed therein to create hypochlorous acid and/or hypobromous acid so that the water is purified and sterilized, a potential difference measuring unit is interposed between a measuring electrode dipped into the water containing the hypochlorous acid and/or hypobromous acid and a reference electrode dipped into water having a prescribed electric conductivity and containing neither hypochlorous acid nor hypobromous acid to thereby evaluate a measured potential difference as a COD value which is an index of pollution. With this arrangement, a polluted state of water can be promptly and simply evaluated as the COD value.