PURPOSE: To prolong the life and prevent noise generation by making a potentiometer have a structure in which no sliding part is used.

CONSTITUTION: A potentiometer is provided with a circular resistor which is installed in a substrate 2 concentrically with a rotary center of a rotary axis 4 and has a cut part in a part, a circular conductor which is installed concentrically with the resistor at a prescribed gap to the inner circumference of the resistor, and a circular photoconductor which is installed in the gap between the conductor and the resistor and becomes an insulating material with high resistance in the condition in which no light is radiated and becomes a conductive material with low resistance in the condition in which light is radiated. The potentiometer is provided also with a light-guiding member 5 to guide light rays emitted out of an electroluminescent diode 10 in order to radiate the light rays to a site corresponding to the rotary angle of the rotary axis 4 in the photoconductor, the diode 10 is installed so as to rotate unitedly with the rotary axis 4.