To provide an underwater oxygen dissolution apparatus and an underwater oxygen dissolution method using the same, which has excellent formation efficiency of air bubbles in fluid while capable of suppressing energy consumption, and can dissolve oxygen contained in the air bubbles into a large amount of fluid in a short time.SOLUTION: An underwater oxygen dissolution apparatus includes: a peripheral wall 2 having a flow change portion 2a for changing a flow direction of a part of first fluid 50； a bottom portion 3 surrounded by the peripheral wall 2； a box-type tank 5 comprising a drop hole 4 provided in the bottom portion 3； a first injection pipe 6 for injecting the first fluid 50 into an interior 5a of the box-type tank 5； a second injection pipe 7 for injecting second fluid 51 into the interior 5a； and a downcomer 8 communicating with the drop hole 4. In the first injection pipe 6, a first opening 6a opens above the inner side of the flow change portion 2a. In the second injection pipe 7, a second opening 7a opens above the drop hole 4.SELECTED DRAWING: Figure 1【課題】消費エネルギーを抑制可能でありながら、流体中での気泡の形成効率が良好であって、この気泡に含まれる酸素を大量の流体の中へ短時間に溶解させることが可能な水中酸素溶解装置およびこれを用いた水中酸素溶解方法を提供する。【解決手段】第１の流体５０の一部の流れ方向を変化させる流れ変化部２ａを有する周壁２と、周壁２で囲まれる底部３と、底部３に設けられる落下孔４を備える箱型タンク５と、第１の流体５０を箱型タンク５の内部５ａに注入する第１の注入管６と、第２の流体５１を内部５ａに注入する第２の注入管７と、落下孔４に連通する下降管８を備え、第１の注入管６は、第１の開口部６ａが流れ変化部２ａの内側の上方に開口し、第２の注入管７は、第２の開口部７ａが落下孔４の上方に開口する。【選択図】図１
