A signal frequency band control system comprises an amplifier for amplifying an input signal. A first differentiation or integration circuit receives an output signal from the amplifier and carries out a primary differentiation or primary integration. A second differentiation or integration circuit is supplied with an output signal from the amplifier and carries out a secondary differentiation or secondary integration. A first gain constant circuit multiplies the output signal from the second differentiation or integration circuit by a first gain constant. A second constant circuit multiplies, by a second gain constant, the signal resulting from the addition of the output signal of the first differentiation or integration circuit and the output signal of the first gain constant circuit. A circuit negatively feeds back, to the amplifier, a signal resulting from the addition of the output signal of the amplifier and the output signal of the second gain constant circuit. The gain constant of one of the first and second gain constant circuits is varied. The cutoff frequency of the frequency band of the output signal of the amplifier is varied in accordance with the variation of the gain constant thus varied.