Provided is a motion assisting device capable of assisting a periodical walking motion of a creature so as to conduct a smooth motion of a leg thereof. According to the motion assisting device, an assisting force applied to the creature so as to assist a leg of the creature in moving with respect to a body thereof is reinforced by an increment in an auxiliary coefficient increased in a reinforcement duration. Accordingly, the stretch motion of the leg at the on-ground state is assisted by a relatively stronger assisting force. Thereby, the body is conducted to translate forward by a floor reaction force subjected to the leg at the on-ground state. A flexion motion of the leg at an off-ground state is induced by a reflex (stretch reflex) to the stretch motion of the leg by the assisting force at a previous off-ground state.