An image forming apparatus, comprising: a light emitter to radiate light to a recording medium; light receivers (22, 23, 24) each including a plurality of detectors to receive a portion of the light radiated to the recording medium and to respectively output independent detection results; and an output determiner that functions as a paper type determiner and an abnormality determiner to determine a change in a conveying state of the recording medium and a relative arrangement between the light emitter and the light receivers (22, 23, 24), based on whether it is determined that data representing a light amount output by the light receivers is normal, according to the independent detection results respectively output by the plurality of detectors, wherein the light receivers (22, 23, 24) include: a transmitted light receiver to detect an amount of light transmitted by the recording medium from the light radiated by the light emitter, a specularly reflected light receiver to detect an amount of light specularly reflected by the recording medium from the light radiated by the light emitter, and a diffusively reflected light receiver to detect an amount of light diffusively reflected by the recording medium from the light radiated by the light emitter, wherein the independent detection results respectively output by the plurality of detectors are light receiving amounts detected by the plurality of detectors or degrees of relative change of the light receiving amounts, wherein when the light receiving amounts detected by the plurality of detectors are normal, the output determiner determines the paper type using the normal data, and wherein when a sum of the light receiving amounts of the plurality of detectors of a light receiver exceeds an upper bound of a range of a preset total light amount threshold or is less than a lower bound of the range of the preset total light amount threshold, the output determiner determines an error in at least one of the conveying state of the recording medium and the relative arrangement between the light emitter and the light receivers (22, 23, 24). a light emitter to radiate light to a recording medium; light receivers (22, 23, 24) each including a plurality of detectors to receive a portion of the light radiated to the recording medium and to respectively output independent detection results; and an output determiner that functions as a paper type determiner and an abnormality determiner to determine a change in a conveying state of the recording medium and a relative arrangement between the light emitter and the light receivers (22, 23, 24), based on whether it is determined that data representing a light amount output by the light receivers is normal, according to the independent detection results respectively output by the plurality of detectors, wherein the light receivers (22, 23, 24) include: a transmitted light receiver to detect an amount of light transmitted by the recording medium from the light radiated by the light emitter, a specularly reflected light receiver to detect an amount of light specularly reflected by the recording medium from the light radiated by the light emitter, and a diffusively reflected light receiver to detect an amount of light diffusively reflected by the recording medium from the light radiated by the light emitter, wherein the independent detection results respectively output by the plurality of detectors are light receiving amounts detected by the plurality of detectors or degrees of relative change of the light receiving amounts, wherein when the light receiving amounts detected by the plurality of detectors are normal, the output determiner determines the paper type using the normal data, and wherein when a sum of the light receiving amounts of the plurality of detectors of a light receiver exceeds an upper bound of a range of a preset total light amount threshold or is less than a lower bound of the range of the preset total light amount threshold, the output determiner determines an error in at least one of the conveying state of the recording medium and the relative arrangement between the light emitter and the light receivers (22, 23, 24). a transmitted light receiver to detect an amount of light transmitted by the recording medium from the light radiated by the light emitter, a specularly reflected light receiver to detect an amount of light specularly reflected by the recording medium from the light radiated by the light emitter, and a diffusively reflected light receiver to detect an amount of light diffusively reflected by the recording medium from the light radiated by the light emitter, wherein the independent detection results respectively output by the plurality of detectors are light receiving amounts detected by the plurality of detectors or degrees of relative change of the light receiving amounts, wherein when the light receiving amounts detected by the plurality of detectors are normal, the output determiner determines the paper type using the normal data, and wherein when a sum of the light receiving amounts of the plurality of detectors of a light receiver exceeds an upper bound of a range of a preset total light amount threshold or is less than a lower bound of the range of the preset total light amount threshold, the output determiner determines an error in at least one of the conveying state of the recording medium and the relative arrangement between the light emitter and the light receivers (22, 23, 24).