PURPOSE: To provide a collision preventing device which is constituted to stop operation and previously transmit an alarm when a danger zone is entered so that booms are prevented from being brought into contact with each other and induction of the occurrence of an accident when a plurality of jib type tower cranes are installed close to each other and a work to suspend and erect a material is effected.

CONSTITUTION: The rise and fall limit angle and the slewing limit angle of a jib corresponding to the mutual entrance prohibition region of jig type simple tower cranes installed in the vicinity with each other are set. A device to the rise and fall angle and the slewing angle is arranged at each two crane. Based on detecting signals from respective detecting devices 2 and 7, operation is brought into a stop and simultaneously, an alarm oscillating device 10a comprising a bell, a buzzer, a display lamp is operated.