PURPOSE: To reduce the cost of manufacturing an offshore working platform by a method in which a portion to share reaction which acts on cords constituting the support legs of the platform is provided in all circumferential direction and the supporting structure of the legs is simplified.

CONSTITUTION: A platform 1 having an upper guide 15 and a lower guide 16 as horizontal reaction-sharing parts is provided in a vertically movable manner along plural legs 4. The leg 4 is formed by combining at least three cords in parallel by supporting parts into a tower form, a projection part 27 is provided to a plate 25 provided to the cords 10, and a part to generate reaction, or a contact piece 32, is provided close to the projection part 27. When tidal wave force and wind force act on the platform, the cords 10 are each supported and the relative movement of the platform 1 and the legs 4 in any direction can therefore be suppressed.