A subsea buoy (Sa) is provided with: a power generation coil portion (20a) formed by winding coils (22a) around a plurality of coil cores (21a); a housing (31a) in which the power generation coil portion (20a) is housed; a magnet array (5a) comprising a plurality of magnets (14a) disposed on the opposite side to the power generation coil portion (20a) with a part of a housing wall (13a) in between so as to be able to be magnetically coupled to the power generation coil portion (20a), said housing wall constituting the housing (31a); a magnet holding member (15a) holding the magnet array (5a); and a water wheel (4a) having a rotational shaft (41) formed to be rotatable by the flow of water. The part of the housing wall (13a) is formed of a nonmagnetic and insulating material or a high electrical resistivity material, and the magnet holding member (15a) is fixedly connected to the rotational shaft (41).
