PURPOSE: To provide arm drive mechanism capable of high speed action and low-priced without shortening machinable workpiece dimension in a workpiece loader of plural arm structure.

CONSTITUTION: A moving body 4 is provided with one motor 16 for advancing/ retreating arms, two arms 11a, 11b, a shift means 17, and brakes 15a, 15b for fixing the respective arms. Two arms 11a, 11b provided at the moving body 4 are driven by one motor 16 by switching the drive mechanism 14a, 14b of two arms 11a, 11b by a shifting device 17. The drive mechanism 14a, 14b of the arms detached from the motor 16 are fixed by the brakes 15a, 15b so as to guarantee the engaged relation at the time of the drive mechanism of the arms being linked again with the motor 16 and to prevent the arms, detached from the motor 16, from moving by their own weight or the like.