To improve the workability when a crane boom is decomposed and assembled, by installing a footboard and a handrail to the crane boom easily, in the condition maintaining their interval adequately.

Plural brackets 33 are fixed to a footboard 32, a stay 34 is installed to a support tube 33C separating as an interval B in the width direction of the footboard 32, and by extending a rope 35 between the mooring rings 34A of the stay 34, the system is composed to maintain a specific interval A between the footboard 32 and the rope 35 constantly. Consequently, a complicated work to regulate the interval of the footboard 32 and the brackets can be made unnecessary, compared when the footboard 32 and the brackets are installed separately to an intermediate boom, for example. As a result, the time necessary for the installing work of a catwalk device 31 can be reduced, and the workability when the crane boom is decomposed and assembled can be improved.