1. A control apparatus of an elastic actuator drive mechanism, comprising: an abnormality determination unit that determines whether or not an output measurement unit that measures an output of an elastic actuator is abnormal; a normal case operation control unit that controls the elastic actuator drive mechanism in accordance with an output of the output measurement unit when the abnormality determination unit determines that the output measurement unit is normal; an abnormal case operation control unit that controls the elastic actuator drive mechanism in accordance with an internal state model when the abnormality determination unit determines that the output measurement unit is abnormal; and a control unit that operates the normal case operation control unit when the abnormality determination unit determines that the output measurement unit is normal, and that switches from the normal case operation control unit to the abnormal case operation control unit so as to operate the abnormal case operation control unit when the abnormality determination unit determines that the output measurement unit is abnormal; wherein the normal case operation control unit comprises: a first desired output unit that outputs a desired value of the output of the elastic actuator; and a torque control unit that calculates a desired joint torque in accordance with an output of the first desired output unit and the output of the output measurement unit, the normal case operation control unit controls the elastic actuator drive mechanism in accordance with the desired joint torque calculated by the torque control unit, the abnormal case operation control unit comprises: a second desired output unit that outputs a desired value of the output of the elastic actuator; and a desired internal state information acquisition unit that acquires desired internal state information on the elastic actuator in accordance with a desired value of the output of the second desired output unit and the internal state model, and the abnormal case operation control unit controls the elastic actuator drive mechanism not in accordance with a measurement result of the output of the elastic actuator but in accordance with the desired internal state information acquired by the desired internal state information acquisition unit so that the elastic actuator is operated continuously.