To provide a system for preventing deposition of underwater dissolved matter on the surface of wetting sections of an underwater installed metallic mechanical structure which makes it possible to obtain a permanent deposition preventive effect at a low cost without requiring painting on the surface of the wetting sections and high cost coating and without separately installing filter installation facilities, etc.

A plus side output terminal 5 of a variable constant-voltage DC power unit 4 is connected to a conductive aerial lead-out section 2 of the underwater installed metallic mechanical structure 1 and a minus side output terminal 6 of the variable constant-voltage DC power unit 4 is connected to a minus side electrode 3 disposed oppositely in water in a contactless state with the underwater installed metallic mechanical structure 1 and the system is so constructed to cause the electrically positive electrification of the underwater installed metallic mechanical structure 1 and to cause the electrically negative electrification of the minus side electrode 3 by feeding a power source from the variable constant-voltage DC power unit 4.