The motion assist device (200) is provided with an auxiliary oscillator generation element (150) configured to generate, on the basis of a second intrinsic angular velocity (ω2) set according to a first oscillator (ξ1) generated from a first motion oscillator (φ1) and a first model and a second oscillator (ξ2) generated from a second motion oscillator (φ2) and a second model, an auxiliary oscillator (η) which includes therein a first auxiliary oscillator (η1) denoting an elastic force originated from a virtual elastic element for assisting the motion of the user so as to approximate a value of a third motion oscillator (φ3) to a desired value (φ0+, φ0−) related to a desired motion scale of the user, and an auxiliary oscillator regulation element (160) configured to sequentially regulate the first auxiliary oscillator (η1) so as to approximate a motion index value of the user to a reference value.