Abstract: Disclosed herein is a tidal current power generator having an underwater connecting structure, which is capable of ensuring the promptness, correctness and safety of an electrical coupling without support by a diver. The tidal current power generator includes: a nacelle in which a turbine rotor and a power generator are installed; and a tower which is coupled to or decoupled from the nacelle. A plug connector is included in the tower. The nacelle includes a hollow tube forming a passage in which the plug connector is inserted and being filled with a nonconductive filler, a socket connector coupled to the inside of the hollow tube and connected to the power generator, and a check valve which is installed in the passage of the hollow tube and prevents the filler from escaping from the hollow tube when the plug connector is not inserted in the hollow tube.