an insulating substrate having an upper surface formed with a strip-like resistor film;

a rotor overlapping the insulating substrate from above; and

a holder held in electrical contact with the rotor for pressing, from an outside, the rotor in a horizontally rotatable manner;

wherein the resistor film has a non-linear configuration with an arcuate portion surrounding a rotation center of the rotor and with a first and a second ends extending toward an edge of the insulating substrate, wherein the rotor includes a contact portion for contact with the resistor film and an engagement portion to which a driver for rotational operation is to be fitted, the rotor being so held that only the contact portion comes into contact with the resistor film,

wherein the resistor further comprises: a first electrode electrically connected to the first end of the resistor film; a second electrode electrically connected to the second end of the resistor film; and a third electrode electrically connected to the rotor, the first, the second and the third electrodes being exposed to the outside at a peripheral surface of the insulating substrate; and

wherein the holder is formed from a conductive metal plate and comprises at least a pair of holding portions extending along an opposite pair of outer side surfaces of the insulating substrate onto a lower surface of the insulating substrate for attaching the holder to the insulating substrate and for pressing and holding the rotor, at least one of the holding portions serving as the third electrode.