PURPOSE: To improve the stability of a clamping effect by shortening the time for arithmetic operation without lowering estimation precision as much as possible by completing discretion on the first-order stage of a state equation at the time of observer estimation matrix calculation.

CONSTITUTION: On the high-order side of an observer control and arithmetic unit 3, a load inertia value JL is calculated by a load inertia arithmetic mechanism 1 based on a joint angle 22 of each rod. On the low-order side of the observer control and arithmetic unit 3, observer estimation matrixes J, F, G and L transferred from the high-order side, torsion angle feedback gain KGs, load velocity feedback gain KWL, velocity signal 28 from the low-order side of the controller and torque command 29 are received, the torsion angle and the load velocity are estimated and multiplied with the respective feedback gains and a torque constant 30 is multiplied to a value subtracted from the torque command value 29 outputted from a position/velocity loop by a subtracter 25 so that current commands I1, I2...In of respective rods can be provided.