PURPOSE: To enable a platform to be set without the need of a floating crane, by jacking up the platform and fixing it with lower legs, after the platform is conducted on the lower legs exposed from the water in site.

CONSTITUTION: After a platform 1 is conducted on the upper section of a jacket 5 set in site, upper legs 6b are welded on lower legs 6a to be firmly fitted. After that, along the upper legs 6b, the platform 1 is lifted up with a jack 7, and the stopper pins 8 of elbow members 3 of the platform 1 are inserted into the stopper slots 9 of the upper legs 6b, and the setting of the platform 1 is completed. As a result, the floating crane of large type is not needed, and a construction cost can be reduced.