A sensor apparatus comprising: an irradiation system (11, 12) provided with a light source and configured to emit linearly polarized light of a first polarization direction onto a sheet-like object, in a direction oblique to a direction orthogonal to a surface of the object; a first photodetector arranged on an optical path of light that is emitted from the irradiation system (11, 12) and then is reflected at the object by regular reflection; a first optical element, arranged on an optical path of light reflected by diffuse reflection from an incidence plane of the object, configured to transmit linearly polarized light of a second polarization direction that is orthogonal to the first polarization direction; a second photodetector configured to receive light passed through the first optical element; a detection unit configured to detect information that is indicative of at least one of basis weight and thickness of the object; wherein the detection unit includes a third photodetector configured to receive light emitted from the irradiation system (11, 12) that passes through the object, and a mechanical thickness sensor configured to detect thickness of the object; the sensor apparatus further comprising a processor (110, 2090) configured to identify the object based on output from the plurality of photodetectors and output from the detection unit, wherein the processor (110, 2090) estimates thickness of the object based on output from the mechanical thickness sensor, and estimates basis weight of the object based on output from the third photodetector when estimated thickness is equal to or less than a prescribed value. an irradiation system (11, 12) provided with a light source and configured to emit linearly polarized light of a first polarization direction onto a sheet-like object, in a direction oblique to a direction orthogonal to a surface of the object; a first photodetector arranged on an optical path of light that is emitted from the irradiation system (11, 12) and then is reflected at the object by regular reflection; a first optical element, arranged on an optical path of light reflected by diffuse reflection from an incidence plane of the object, configured to transmit linearly polarized light of a second polarization direction that is orthogonal to the first polarization direction; a second photodetector configured to receive light passed through the first optical element; a detection unit configured to detect information that is indicative of at least one of basis weight and thickness of the object; wherein the detection unit includes a third photodetector configured to receive light emitted from the irradiation system (11, 12) that passes through the object, and a mechanical thickness sensor configured to detect thickness of the object; the sensor apparatus further comprising a processor (110, 2090) configured to identify the object based on output from the plurality of photodetectors and output from the detection unit, wherein the processor (110, 2090) estimates thickness of the object based on output from the mechanical thickness sensor, and estimates basis weight of the object based on output from the third photodetector when estimated thickness is equal to or less than a prescribed value.