PURPOSE: To improve gripper positioning accuracy by storing a gripper main body loosely in a storage cylinder and providing the gripper main body with an eccentricity detecting means which detects relative displacement from the axial center of the storage cylinder.

CONSTITUTION: The eccentricity detecting means (displacement detector) 17 is formed by arranging guide rollers which are energized radially by springs toward the internal wall surface of the storage cylinder 4 dispersedly at six positions on the periphery of the gripper main body 3 and detects the relative eccentricity quantity and eccentricity direction of he gripper main body 3. Fur ther, a displacement detector 17 detects the displacement of the gripper main body 3 and a data processing part 24 converts the displacement into the expansion/contraction and turning correction quantity of a swing arm mechanism 2, which is fed back to its driving system. The swing arm mechanism 2, on the other hand, adjusts the swing arm finely in the turn and expanding/ contracting direction by regarding said feedback quantity as control deviation to correct and move the storage cylinder 4 of the gripper to an accurate position about a specified reactor column.