PURPOSE: To obtain a wavelength/voltage conveter having a simple structure and low cost, by means of being constituted by the switching circuit, the constant current circuits the 1st and 2nd capacitors, the diode and the output circuit each.

CONSTITUTION: The converter consists of the following units: the input amplifying stage comprising the transistor TR1, resistances R11WR14 puls capacitors C11 and C12; the switching stage consisting of the transistor TR2; the constant current circuit comprising the 1st and 2nd capacitors C21 and C51, the diode D connected between the C21 and C51, trnasistors TR3 and TR4 plus resistances R31, R41 and R42 each; and the output stage comprising the transistor TR5, resistances R51 and R52 plus the capacitor C61 and forming the emitter-follower respectively. In such constitution, a simple structure and low cost can be realized for a wevelength/voltage converter.