Abstract: A case of approximate sphere is formed by assembling a pair of hemispherical shell pieces with elastic materials. A rack, capable of shifting in the axial direction in connection with shift of the shell pieces, is supported within the case. A pinion designed to rotate in engagement with the rack is provided within the case. A grip indicator dial designed to rotate in connection with the pinion is provided within the case. This grip indicator dial is provided with a scale observable from outside the case. Within the case is provided a target-grip setting dial capable of being operated from outside the case. The target-grip setting dial is provided with a scale indicating the target-grip.