PURPOSE: To provide an inserting device wherein a small-sized connecting boom can be inserted into a large-sized connecting boom by crane operation without neccesitating any replacement of slinging ropes and any manual press-in work.

CONSTITUTION: One end of a small-sized connecting boom 9 to be inserted into a large-sized connecting boom 8 is connected to a vertical frame portion 1 serving as a component of a suspending apparatus H so that the boom 9 may be held horizontally. The connecting boom 9 is inserted into the connecting boom 8 by suspending a horizontal frame portion 2 serving as a component of the suspending apparatus H by means of a hook 13 and by moving the hook 13. The part from which the horizontal frame portion 2 is suspended is set above approximately the center of gravity of the connecting boom 9.