To provide a support 1 by installing overhang part supports 10 and 10' opposite to each other between main beams 11 and 11'.

The support 1 is provided with a length-variable strut comprising a length-variable beam comprising a first hollow beam member 12 and a second beam member 13 slidably inserted into the first member, a fixing member for fixing the base end of the first member 12 to the main beam 11, three upper strut members 14b, 14c and 14e, one lower strut member 14a and a joint part 14d, and the top ends of the three upper strut members are joined with the tip end and the base end of the first hollow beam member 12 and the tip end of the second beam member 13. The overhang part supports 10 are installed opposite to each other with spacing the tip ends of the beams apart to each other between the main beams 11 and 11', and a cover member 80 is placed to cover the two tip ends of the beams.