To transmit electric power not requiring a maintenance over a long year and generate an electric energy with a compact structure efficiently.

A power generating mechanism is arranged horizontally in the hollow vessel 12 with a sealed up structure moored on the sea bottom by a flexible mooring member 13 swung on the sea surface 11 by an undulation energy. In its inside, a long sideways cylinder 19 in which a piston 23 interlocking with the swing of the hollow vessel 12 and reciprocating horizontally is built, two cylinder chambers 24, 25 partitioned by the piston 23 of the long sideways cylinder 19, rotary bodies 21, 22 connected communicatingly through a sealed up fluid circuit and rotated by the movement of an operation fluid by the reciprocating of the piston 23 and a generator 20 for carrying out the power generation by the rotation of the rotary bodies 21, 22 are provided.