Abstract: An oxygen concentration sensor trouble discriminating apparatus in which a counter executes a counting operation every time a lean state of an air-fuel ratio is detected on the basis of an output signal of an oxygen concentration sensor for the first time after a feedback control operating state was not detected. In the case where the air-fuel ratio feedback control operating state is detected, if the output signal of the oxygen concentration sensor indicates an inversion of the rich and lean states of the air-fuel ratio, a count value of the counter is reset to an initial value. When the count value is larger than a predetermined value, the oxygen concentration sensor is decided to be in trouble.