timing previously memorized in the ram 204 with an output voltage v 02 n of the first oxygen sensor 10 which is currently detected and determines the comparison result when v 02 n lt v 02 n 1 the operation determines that the output voltage v 02 of the first oxygen sensor 10 is decreasing and introduces k fl to the digital low pass filter coefficient k f in step s31 when v 02 n u2267 v 02 n 1 the operation determines that the output voltage v 02 of the first oxygen sensor 10 is increasing and introduces k fr to the filter coefficient k f in step s32 and proceeds to the digital low pass filter treatment in step s15 next a fourth embodiment of the present invention will be explained referring to fig1 and 14 fig1 is a control block diagram of the fourth embodiment in fig1 a reference numeral 28 designates an increase or decrease determining means which determines whether the filter output voltage v 02f is in the direction of increasing or in the direction of decreasing the low pass filter 22a is a low pass filter having two time constants which are set based on the air fuel ratio correction signal k fb2 and the time constant are switched by a determining signal k 3 of the increase or decrease determining means 28 the other construction is.