A highly accurate vector absolute-value calculation circuit uses analog processing and minimal hardware. Signal voltages corresponding to an I component (real number part) and a Q component (imaginary number part) are input to a first absolute-value calculation circuit 13 and a second absolute-value calculation circuit 14 through terminals 11 and 12, respectively, and they are each converted into absolute-value signals. The component I absolute-value and component Q absolute-value are compared in a comparison circuit 20. According to the result, the larger absolute-value signals are output to an input capacitor 23 of a neural computation circuit, and the smaller absolute-value signals are output to an input capacitor 24 by controlling multiplexers 21 and 22. The capacity ratio of a feedback capacitor 26 of a neural computation circuit and input capacitors 23 and 24 is 11:10:5. The complex number absolute-value calculated by the following formula is output from an output terminal 27. ##EQU1##