To enable the automatic aeration without requiring the supply from a site on the land and selecting a site such as the offing by using the natural clean energy as a driving source.

This automatic aerator on the sea is obtained by installing a solar cell 1, an inverter 5 for converting a DC current generated in the solar cell 1 into an AC current, a compressor 6, operable with the AC current after the conversion with the inverter 5 and producing compressed air, a diffusing pipe 7 for feeding the compressed air fed from the compressor 6 into water or the sea and a storage battery or cell 4 for storing the electric power from the solar cell 1 in a boat, etc.