To provide an index conversion circuit in which a variation in temperature or dimensions of circuit elements has no effect on the index conversion characteristics of output current.

Based on a gain being controlled by a gain control signal VG, reference voltages Vref1 and Vref2 are converted by a voltage conversion circuit 1A into differential output voltages Vd1 and Vd2 being inputted to an active impedance bridge 2. In the active impedance bridge 2, equilibrium is sustained by the ratio (Vg4/Vg3) of the Vg3 generated by an amplifier 25 and the inclination control voltage Vg4 of index conversion characteristics and the gain control signal VG is settled. A voltage conversion circuit 1B having a gain being controlled by the gain control signal VG converts the reference voltage Vref1 and a control voltage Vc into a differential output voltage Vd3 being inputted to an index conversion element 3. Consequently, inclination in the index conversion characteristics of the output current IOUT from the index conversion element 3 for the control voltage Vc is determined depending on Vg4/Vg3.