PURPOSE: To provide a piping device in which hydraulic pipes respectively attached to two arms which are pin-joined, are connected with no use of a hydraulic hose, and to decrease design change as far as possible.

CONSTITUTION: A rotary joint 12 fitted in a cylindrical support pin 8a with which arm members 1a, 1b are pivotally joined, is formed with a cylindrical part 12b and a columnar part 12a which are rotated relative to each other. The columnar part 12a is fixed to the arm member 1a, or the columnar part 12b is secured to the arm member 1b, and hydraulic passages 12DU, 12dl are communicated with each other by means of openings 12D, 12d. Further, hydraulic passages 12EU, 12eL are communicated with each other by means of openings 12E, 12e at least one of which is formed in a circumferential groove (which is not shown). The hydraulic passages 12DU and 12EU are connected to and hydraulic pipes 11RL, 10RL laid on the arm member 1a, and further, hydraulic pipes 11RM and 10RM are connected to hydraulic pipes 11RM and 10RM laid on the arm member 1b.