PURPOSE: To maintain ballance continuously thus to enable safe work by coupling a weight truck movable on a rail laid in parallel with one side and a working machine movable freely at the other side through a wire rope.

CONSTITUTION: A rail 2 is laid in parallel with the side of platform near to one side of upper deck of platform 1 while a truck 3 mounted with a weight 4 is arranged movably. A working machine 5 is mounted at the center near to the other side of platform 1 movably back and fro in parallel with the rail 2 to face the work machine 5 and the truck 3 each other. Upon moving of the working machine 5, the weight mounting truck 3 moves in reverse direction through a wirerope 8 and pulley 6 to maintain ballance of working platform continuously resulting in safety work.