A sensor apparatus comprising: an irradiation system provided with a light source and configured to emit linearly polarized light of a first polarization direction onto a sheet-like object (M), in a direction oblique to a direction orthogonal to a surface of the object (M); a first photodetector arranged on an optical path of light that is emitted from the irradiation system and then is reflected at the object (M) 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 (M), 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 (M); wherein the detection unit includes a third photodetector configured to receive light emitted from the irradiation system that passes through the object (M), and a mechanical thickness sensor configured to detect thickness of the object (M); the sensor apparatus further comprising a processor configured to identify the object (M) based on output from the plurality of photodetectors and output from the detection unit , wherein the processor estimates thickness of the object (M) based on output from the mechanical thickness sensor , and estimates basis weight of the object (M) based on output from the third photodetector when estimated thickness is equal to or less than a prescribed value. an irradiation system provided with a light source and configured to emit linearly polarized light of a first polarization direction onto a sheet-like object (M), in a direction oblique to a direction orthogonal to a surface of the object (M); a first photodetector arranged on an optical path of light that is emitted from the irradiation system and then is reflected at the object (M) 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 (M), 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 (M); wherein the detection unit includes a third photodetector configured to receive light emitted from the irradiation system that passes through the object (M), and a mechanical thickness sensor configured to detect thickness of the object (M); the sensor apparatus further comprising a processor configured to identify the object (M) based on output from the plurality of photodetectors and output from the detection unit , wherein the processor estimates thickness of the object (M) based on output from the mechanical thickness sensor , and estimates basis weight of the object (M) based on output from the third photodetector when estimated thickness is equal to or less than a prescribed value. <EOS>