1. An air-fuel ratio control apparatus comprising: an air-fuel ratio sensor having a solid electrolyte layer and at least a pair of electrodes provided on both sides of said solid electrolyte layer, said air fuel ratio sensor being constructed to generate an electromotive force which changes stepwisely in a zone in close proximity to a stoichiometric air-fuel ratio point when no forced current is supplied thereto externally; sensor characteristics changing means for changing electromotive force generating characteristics of said air-fuel ratio sensor to characteristics with an air-fuel ratio point, at which said electromotive force changes stepwisely, shifted from said stoichiometric air-fuel ratio point to one of a lean and a rich zone by a forced current from one of said electrodes to the other electrode of said air-fuel ratio sensor; and air-fuel ratio control means for implementing feedback control of an air-fuel ratio by using said electromotive force characteristics changed by said sensor characteristics changing means, said changed electromotive force generating characteristics being used for determining whether an actual air-fuel ratio is in said one of a lean and a rich zone.