PROBLEM TO BE SOLVED: To evenly irradiate a seedling under cultivation with light even if the size of a culture tank is increased. SOLUTION: Aquaculture seawater 51 used for culturing a seedling 31 in which spores of seaweed or germinated spores are aggregated is injected into a culture tank 111, and the seedling 31 is released into the culture tank 111 and floats in water. The stirrer 201 is thrown into the aquaculture seawater 51 and fixed in position, and the agitator 201 is driven to rotate the water scraping blades 251 that rotate around the vertical axis to generate a vertical swirling flow in the aquaculture seawater 51. The aquaculture tank 111 has a circular shape as an example, and the maximum depth of the aquaculture seawater 51 does not exceed 600 mm. As the aquaculture seawater 51, for example, underground seawater or drainage obtained by cultivating seafood or the like on land is used. [Selection diagram] FIG.【課題】養殖槽の大きさが大きくなったとしても、養殖中の種苗に均等に光照射を行い得るようにすること。【解決手段】海藻の胞子又は胞子が発芽した発芽体が集塊化した種苗３１の培養に用いる養殖用海水５１を養殖槽１１１に注入し、養殖槽１１１に種苗３１を放ち、水に浮遊する攪拌機２０１を養殖用海水５１に投入して位置固定し、攪拌機２０１を駆動して垂直軸周りに回転する水掻き羽根２５１を回転させ、養殖用海水５１に縦方向の旋回流を生じさせる。養殖槽１１１は一例として円形状であり、養殖用海水５１の最大水深は６００ｍｍを越えないようにする。養殖用海水５１には、例えば地下海水、あるいは魚介類等を陸上養殖した排水を用いる。【選択図】図２
