A sensor apparatus of the present invention is provided with a failure diagnosis circuit for setting as a failure diagnosis object section at least any one of a drive circuit section, a detection device, a detection circuit section and a processing circuit section, determining whether the failure diagnosis object section is normal or abnormal, and outputting a failure detection signal from a second output terminal in the case of determining abnormality. It is configured such that a signal of a value outside a range of a normal output voltage is outputted from the first output terminal in the case of the failure diagnosis circuit determining abnormality of the failure diagnosis object section, thereby to improve reliability under abnormal condition.