PURPOSE: To effectively remove oxygen dissolved in water, by providing the source of a suboxide ion reactive with the dissolved oxygen in the passage of equipment through which service water for cooling or heating is circulated and the anion-exchange resin which can adsorb formed anions.

CONSTITUTION: Cation-exchange resin 4a and anion-exchange resin 4b are received in an ion-exchange means 4. A deoxidizing means 5 is packed with the other anion-exchange resin 5a to which sulfurous ion is adsorbed. Metal and contaminating ions dissolved in cooling water are removed by the ion-exchange resin-packed means 4. A very small amount of dissolved oxygen existent in the cooling water is fixed as sulfuric ion by the reaction formed when the cooling water comes in contact with the anion-exchange resin poured in the deoxidizing means 5 and adsorbing sulfurous ion to it, and removed from the cooling water by adsorption with the anion-exchange resin 5a.