PURPOSE: To apply a stable control voltage to a circuit to be controlled by devising the title circuit such that a dead zone is set to a control voltage output circuit at which a constant control voltage is outputted from an output section of the control voltage output circuit even if a slider terminal of a variable resistor is in any position.

CONSTITUTION: The dead zone setting circuit CC is provided, in which 1st and 2nd voltage clip transistors(TRs) 1, 2 whose emitters are connected in common and of opposite polarity to the 2nd input section a2 of a differential amplifier AA, 1st and 2nd reference power supplies V1, V2 connecting to bases of the TRs 1, 2 and a resistor R6 are provided. Then a dead band is set to the control voltage output circuit and even when a slider (t) of the variable resistor VR is set to any position, a constant control voltage is outputted from an output section of the control voltage output circuit at the dead band. Thus, a stable control voltage is applied to a circuit BB to be controlled.