PURPOSE: To easily connect the legs of a platform with the struts of a marine structure in a short time by providing a guide piece with a vertical groove on the inside of an opening and a fixing piece of a circular form to be inserted into the vertical groove.

CONSTITUTION: In the strut 3 of a marine structure 1, a vertical hole 33 to be fitted with the leg 5 of a platform 4 is provided at the center, and also an opening 32 provided on the side of the vertical hole 33 is provided on the upper part. Then, a guide piece 31 with vertical grooves 34 and 34a is provided in the inside of the opening 32, and a circular fixing piece 35 to be fitted into the vertical grooves 34 and 34a is also provided in the strut 3. By this, the connection of the legs 5 of the platform 4 and the strut 3 of the marine structure 1 can be easily attained in a short time without being affected by weather and oceanic conditions.