The present invention relates to a method and apparatus, in which the quality of a large amount of water and/or the quantity of dissolved oxygen in the water is improved. Water in which slaked lime is dissolved in advance and/or water of a high oxygen content which is generated in advance is supplied into water pumped by an intermittent pneumatic water pumping unit so that the water having the slaked lime dissolved therein and/or the water of high oxygen content is diffused in the large amount of water. The present invention also relates to a method and apparatus, in which the quality of a large amount of water and/or the quantity of dissolved oxygen in the large amount of water is improved by water with slaked lime dissolved therein and/or water of high oxygen content being conducted to the bottom of a lake or the like. An intermittent pneumatic water pumping unit is operated in the lake or the like so that the large amount of water is caused to flow up and down and be agitated.