train by the noise removing circuit 27 so that noise ds 1 and ds 2 are removed as shown in fig7 e the signal of fig7 e is fed to a square pulse generator 28 the square pulse generator 28 produces a square output signal shown in fig7 f by triggering with the signal of fig7 e since the low positive excursion 34 e 34 of the mixture in fig7 a is positioned in the lean side a wide low level portion 34 w 34 is formed in the judgement signal of fig7 f thus the fact that the mixture having the waveform of fig7 a is on the lean side can be detected by the lower level portion 34 w 34 of the signal f derived from the oxygen sensor 19 fig9 shows an example of the signal f 39 from the square pulse generator 28 when the air fuel ratio of the mixture is at the stoichiometric value compare the corresponding dither signal a but where the center line 0 has been shifted to the stoichiometric line s the signal from the square pulse generator 28 comprises pulses a 39 to f 39 each having the same pulse width fig1 shows another example of the signal f 34 when the air fuel mixture deviates to the rich side compare the corresponding dither signal but where the center line 0 has been shifted to the rich side r the signal f 34 includes a wide high level portion d 39 e 39 f 39 that is if the positive excursion of the dither signal which corresponds to the air fuel ratio of the mixture deviates from the stoichiometric value a wide high level signal is.