A submersible power generator with an inner/outer double rotational armatures-type power generation mechanism provided with an outer rotational armature and an inner rotational armature opposing the outer rotational armature and rotatable in a direction opposite of the outer rotational armature, and a pair of propellers disposed coaxially with blades of each of the pairs of propellers twists in opposite directions relative to an extending direction of a central axis of the pair of propellers, and a casing for accommodating and shielding the inner/outer double rotational armatures-type power generation mechanism. The submersible power generator, as installed under water to operate in a current, has a buoyancy F acting thereon that is larger than gravity W acting thereon during operation of the submersible power generator due to gravity acting on the mooring wire, so that F>W+gravity acting on the mooring wire−buoyancy acting on the mooring wire.

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