A device for regulating stiffness has a rotary shaft coupled to the driven member to rotate the driven member, a rotational connection body coupled to the rotary shaft to freely rotate thereon and rotating by a driving power of the driving motor, and an elastic connection body fixed to the rotary shaft and extending in the length direction of the rotary shaft to connect to the rotational connection body, wherein the elastic connection body gives an elastic force in a direction opposite to the rotating direction of the rotational connection body to interrupt free rotation of the rotational connection body with respect to the rotary shaft, and wherein if the rotational connection body rotates by the driving motor, the elastic force of the elastic connection body acts as spring so that the rotating force of the rotational connection body is transmitted to the rotary shaft.