Abstract: A water quality control method for recycling cooling water in a cooling tower equipped with a blow system and water treatment agent introduction means. Water is made up in response to the blow-down amount encountered by a blowing operation, and water treatment introduction means is actuated to introduce the water treatment agent into the recycle cooling water in the cooling tower, whereby the concentration of said agent in the recycle cooling water is retained at a desired level. The water treatment agent is made up by a predetermined amount, according to the amount of scattered water, at predetermined intervals of time during the operation of the arrangement. The water treatment introduction means is actuated after lapse of a fixed period of time, based on a predetermined level of the conductivity of the cooling water at the operational starting or cleaning of the cooling tower.