To inexpensively form a corrosion prevention film in a short period, thus to easily apply the same to general-purpose marine steel structures.

In the method for forming a corrosion prevention film, a steel caisson 1 is used as a cathode, an undersea member 3 provided in seawater so as to be confronted with the steel caisson 1 is used as an anode, direct current is passed between the electrodes, and a corrosion prevention film is formed on the steel caisson 1 by the electrolytic reaction in the seawater, so as to perform corrosion prevention, the electric current is passed between the electrodes in such a manner that current density at which the film composed essentially of magnesium hydroxide is formed is controlled, so as to form the film with a prescribed film thickness, thereafter, the feed of the electric current is stopped, and, by compositional substitution action in which calcium carbonate is substituted for the magnesium hydroxide formed in the presence of the seawater, the corrosion prevention film is formed.