A telescopic arm for operating machines, of the type including a tubular outer arm, hinge-articulated to a platform provided in said machine, four coaxial slip-off members having a decreasing tubular section, from the biggest to the smallest one, respectively, first, second, third and fourth member, and an operative head associated at the free end of said fourth slip-off member, with said telescopic arm being tiltable in a vertical plane, from a substantially horizontal position to a nearly vertical one and aimed at assuming a close configuration, in which said slip-off members,,, are introduced one into another, and housed in said outer arm, and an open configuration, in which the same slip-off members are at least partially extended to place the above mentioned operative head in a prefixed working area, with the same telescopic arm including: - a drive system, supported, at least partially, outside of the above mentioned telescopic arm, for moving synchronously and with the same extent said slip-off members,,, in their outgoing and ingoing strokes; - a hydraulic circuit, housed inside said telescopic arm, connected to an outer hydraulic system, for supplying and returning fluid under pressure to and from said operative head, with the above mentioned hydraulic circuit comprising: a first group of hydraulic conduits, in which first flexible pipes are provided, set in communication with said hydraulic system, first rigid pipes, connected in series to said first flexible pipes and extended along said second slip-off member up to its outer head; characterized in that the ends of the first flexible pipes are fastened, respectively upstream and downstream, to said outer arm and to the inner head of said second slip-off member and in that the telescopic arm further comprises: - a first idler pulley supported at the inner head of said first slip-off member, in dynamic engagement with said flexible pipes and aimed at keeping the latter stretched during the ingoing step of the same first and second slip-off member (10, 20); - a second group of hydraulic conduits, connected in series to the first one, the second group including second flexible pipes set in communication with said first rigid pipes and fastened, with the respective ends, upstream and downstream, to said second slip-off member and to the inner head of said fourth slip-off member; a second idler pulley, supported at the inner head of said third slip-off member, in dynamic engagement with said second flexible pipes and aimed at keeping the latter stretched during the ingoing step of the same third and fourth slip-off members,; second rigid pipes, connected in series to said second flexible pipes and extended along said fourth slip-off member up to said operative head. - a drive system, supported, at least partially, outside of the above mentioned telescopic arm, for moving synchronously and with the same extent said slip-off members,,, in their outgoing and ingoing strokes; - a hydraulic circuit, housed inside said telescopic arm, connected to an outer hydraulic system, for supplying and returning fluid under pressure to and from said operative head, with the above mentioned hydraulic circuit comprising: a first group of hydraulic conduits, in which first flexible pipes are provided, set in communication with said hydraulic system, first rigid pipes, connected in series to said first flexible pipes and extended along said second slip-off member up to its outer head; characterized in that the ends of the first flexible pipes are fastened, respectively upstream and downstream, to said outer arm and to the inner head of said second slip-off member - a first idler pulley supported at the inner head of said first slip-off member, in dynamic engagement with said flexible pipes and aimed at keeping the latter stretched during the ingoing step of the same first and second slip-off member (10, 20); - a second group of hydraulic conduits, connected in series to the first one, the second group including second flexible pipes set in communication with said first rigid pipes and fastened, with the respective ends, upstream and downstream, to said second slip-off member and to the inner head of said fourth slip-off member; a second idler pulley, supported at the inner head of said third slip-off member, in dynamic engagement with said second flexible pipes and aimed at keeping the latter stretched during the ingoing step of the same third and fourth slip-off members,; second rigid pipes, connected in series to said second flexible pipes and extended along said fourth slip-off member up to said operative head.