a device for forming a stereophonic sound field in a simulated fashion constructed in the form of a monophonic - to - stereophonic conversion circuit in which a monophonic signal is applied to cascade - connected twin - t circuits having different null point frequencies and an output signal of the twin - t circuits is provided as one stereophonic channel signal whereas an output obtained by substracting the one stereophonic channel signal from the monophonic signal is provided as the other stereophonic channel signal . a low - frequency signal attenuating resistor and a high - frequency signal attenuating filter are provided on the output side of the twin - t circuits to equalize peak values in frequency characteristics of the output signal of the twin - t circuits and thus obtained signal is amplified to be equalized in peak level to the monophonic signal and thereafter is applied to a subtractor together with the monophonic signal . such conversion circuit may be used in a simulated 4 - channel stereophonic reproduction device in which a difference signal is produced from a 2 - channel stereophonic signal and rear 2 - channel stereophonic signals are produced on the basis of this difference signal , and the conversion circuit is applicable to such production of the rear 2 - channel stereophonic signals from the difference signal .