An apparatus for holding a sensor such that the sensor touches an object at an arbitrary position in a three-dimensional space, the apparatus including a base member; a rotatable member which is rotatable relative to the base member about a vertical axis line; a first link device including a first stationary link, a first movable link, and two first pivotable links which are pivotably connected to the first stationary link and the first movable link, such that the first stationary link, the first movable link, and the two first pivotable links cooperate with each other to define a quadrilateral, wherein the first stationary link is fixed to the rotatable member such that the first movable link is movable in a plane containing the axis line; a second link device including two second pivotable links, and a second stationary link and a second movable link which are pivotably connected to the two second pivotable links, such that the two second pivotable links, the second stationary link, and the second movable link cooperate with each other to define a quadrilateral, wherein the second stationary link is fixed to the first movable link such that the second movable link is movable in the plane containing the axis line, and wherein the second movable link supports the sensor; a first biasing device which produces a thrust having a directional component resisting a load applied to the first movable link; and a second biasing device which produces a thrust having a directional component resisting a load applied to the second movable link.