PURPOSE: To prevent the occurrence of an assembling miss when connecting a plurality of extension booms in the specified assembling order and assembling a boom having a desired length by providing identification information memory devices storing the identification information for identifying the types of the extension booms on the extension booms.

CONSTITUTION: When a lower boom 1, extension booms 2, 3, and an upper boom 4 are to be connected to assemble a boom 22 having a desired length, the extension booms 2, 3 are connected according to the specified combination and connecting order while the connecting method specified in a manual is checked. Cables 10 are connected to memory devices 9 provided on the booms 1-4 from the lower number in sequence, and the data on the length and superimposition for identifying the types of the extension booms 2, 3 are sent to a calculating device 8. The combination and connecting order of the extension booms 2, 3 are compared with the reference data stored in a reference data memory section to check whether they coincide or not. In case of no, a safety device is operated and the operation of a crane is prohibited while no warning is generated.