PURPOSE: To make the resistance value of the title resistor adjustable while the voltage value is confirmed by only operating one operating rod by providing a recessed section on a rotatable body and an interior- and periphery-side terminals at the interior and periphery of the recessed section.

CONSTITUTION: A recessed section 7 is formed on a rotatable body 4 for inserting the leading end of an adjustment rod and an interior- and periphery- side terminals 8 and 10 are formed at the interior and periphery of the section 7 by exposing a conductor. One of the terminals 8 or 10 is electrically coupled with a slider 6 which slides on a ring-like resistor 3 and further electrically connected to the second lead pin 16 provided on a base 1 through a shoe 14. The other terminal 10 or 8 is connected to one end 12 of the resistor 3 and, at the same time, electrically connected to the first lead pin 15 provided on the base 1. Therefore, voltage adjustment can be performed easily through a semifixed resistor by only operating one adjustment rod by one hand.