To provide an arm drive device for a robot, which prevents inadvertent rotation of an arm even when a belt of a belt transmission mechanism transmitting the rotation of a motor to an arm side is cut.

Auxiliary pulleys 19, 20 each comprising a toothed pulley are attached, separately from a drive pulley 14 and a follower pulley 15, to a rotating shaft 17 of the motor and to an input shaft 18 of a reduction gear, respectively, and an auxiliary belt 22 comprising a toothed belt is wound, in a loosened state, around both of the auxiliary pulleys 19, 20. If a transmission belt 16 is cut, teeth 22a of the auxiliary belt 22 mesh with teeth 19a of the auxiliary pulley 19 and with teeth 20a of the auxiliary pulley 20 to put the rotating shaft 17 and the input shaft 18 in the state of transmitting power therebetween.