Provided is a device capable of assisting a periodical motion of a creature, such as a human, by applying a force to the creature so as to match a motion rhythm and a motion scale of the periodical motion to a desired motion rhythm and a desired motion scale, respectively, while reducing computation processing load. According to a motion assist device (10) of the present invention, a value of a member or a coefficient contained in a simultaneous differential equation of a state variable (ui) for defining a second model which generates a second oscillator (ξ2) is adjusted by means of an adjusting device (14). The second oscillator (ξ2) is generated by the adjusted second model and a torque (T) applied to a human (P) is controlled to vary periodically according to the second oscillator (ξ2).
