1. An apparatus for controlling an elastic actuator, comprising: an internal state measurement means for measuring an internal state of said elastic actuator, the internal state being changed by drive of said elastic actuator; an output measurement means for measuring an output of said elastic actuator; an output error compensation means for compensating an output error by inputting a desired value of the output of said elastic actuator and a measurement value of the output of said elastic actuator, the measurement value being measured by said output measurement means; a desired internal state determination means for determining a desired value of the internal state of said elastic actuator from the desired value of the output of said elastic actuator and the measurement value of the output of said elastic actuator; and an internal state error compensation means for compensating an internal state error by inputting an output from said output error compensation means, an output from said desired internal state determination means, and an output from said internal state measurement means, the apparatus performing control such that the measurement value of said output of said elastic actuator becomes the desired value of said output based on said internal state error compensated by said internal state error compensation means.