To provide an arm device capable of preventing or restricting the lateral displacement of a work to be transferred placed on a fork member provided at a tip when straightly moving the arm device by restricting the elongation of a toothed belt and the influence of the hysteresis or the like at the minimum.

This arm device is provided with a first arm A1 connoted to a base V, a second arm A2 connected to the first arm A1, and a fork member F connected to the second arm A2. Reduction gears R1-R3 are assembled in each joint part of the three members, and input shafts thereof are connected to each other through a synchronous pulley and a toothed belt, and the first and the second arms A1, A2 and the fork member F are connected to the output shafts thereof.