The invention relates to a sensor apparatus for a self-propelled floor cleaning appliance for detecting a down step, having a transmission unit and a reception unit for infrared radiation and having evaluation electronics for evaluating the intensity of the radiation received. To develop the sensor apparatus in such a manner that a down step or a steep downslope of the floor surface can be determined more reliably, while the sensor apparatus can be produced at low cost, according to the invention it is proposed that the sensor apparatus has the reception unit and a first and a second transmission unit, the second transmission unit being disposed at a distance from a connecting straight line running through the reception unit and the first transmission unit. The invention also proposes a floor cleaning appliance having a sensor apparatus of this type.