PURPOSE: To provide an polygonal line processor by which an original signal can exactly be expanded and reproduced by executing the polygonal line processing which is not affected by temperature or the like without necessitating the expensive polygonal line processor such as an LUT by means of an exact polygonal line processing in which a waveform can not be changed even in the neighborhood of a polygonal point.

CONSTITUTION: This processor is equipped with an amplifier circuit 2 which amplifiers an input signal, a level shift circuit 3 which level shifts the output signal of the amplifier circuit 2 and outputs a level shift output signal, and a comparator circuit 4 which compares the input signal with the level shift output signal and outputs one of them. The comparator circuit 4 is equipped with a pair of first and second tranasistors Q1 and Q2 for comparing both the signals, third and fourth transistors Q3 and Q4 for offsetting the change of voltages between bases and collectors due to the change of the collector currents of the first and second transistors Q1 and Q2, and current mirror circuits 6 and 7 for transmitting the collector current values of the first and second transistors Q1 and Q2 to the third and fourth transistors Q3 and Q4.