To provide a low-cost foundation for a sea wind power plant, capable of reducing sea pollution and seacoast erosion with the construction thereof, and to provide a constructing method thereof.

This foundation for a sea wind power plant has caissons 2 and 3 and a column support 7 fixed to the central part inside of the caissons. After sinking this foundation in the sea bottom, the filling such as a melted slug is thrown into a filling housing space of the caissons, and breakwater blocks 4 are arranged in breakwater block loading surfaces of the caissons and in the periphery of the caissons.