A fixed electrode and a movable electrode to be used to drive each arm are formed at a drive unit. As a voltage is applied between the fixed electrode and the movable electrode, the movable electrode is caused to move by coulomb force, thereby driving the arm 3 in a closing operation. By detecting a change occurring in the electrostatic capacity between the fixed electrode and the movable electrode at this time, a decision can be made as to whether or not the sample has been gripped.