Abstract: A roller is installed in a case having an opening at its upper cover, in such a manner that the outer marginal portion of the roller projects partially through the opening of the case. No return force is applied to the roller at all and the same can be rotated by a foot of a dentist. A variable resistor adapted to transmit the turning movement of such roller is also installed in the case so that the change in resistance occurs in proportion to the turning movement of the roller. A cord of the variable resistor extends out of the case and the forward end of such cord is fitted with a plug.