An abnormality detecting apparatus configured to detect a rolling abnormality on a surface of a metal sheet rolled by a rolling mill, the abnormality detecting apparatus comprising: a camera (81, 82) configured to capture an image of the metal sheet whose abnormality is to be detected; and an image processing section configured to divide luminance data of pixels in a range of the metal sheet from the image captured by the camera (81, 82) into three components of an R value, a G value, and a B value, characterized in that the image processing section is configured to detect a rolling abnormality on the surface of the metal sheet on the basis of a relationship between the luminance data of two components among the luminance data of the three components; and in that the abnormality detecting apparatus further comprises a database configured to store information about a first threshold value boundary determined beforehand for dividing a normal value and an abnormal value from each other from a distribution produced by plotting, from a collected plurality of image data representing normal and abnormal data, on a two-dimensional graph of a combination of two components, luminance data of one component of an R value, a G value, and a B value of the combination of two components and luminance data of another component of the combination of two components, and a second threshold value boundary determined beforehand for dividing a normal value and an abnormal value from each other from a distribution produced by plotting, from the collected plurality of image data, on a two-dimensional graph of another combination of two components, luminance data of one component of the other combination of two components and luminance data of another component of the other combination of two components, wherein the image processing section is configured to detect a rolling abnormality if a detected result obtained from the luminance data of the pixels in the range of the metal sheet of the combination of two components from among the R value, the G value, and the B value and the first threshold value boundary indicates an abnormality and if a detected result obtained from the luminance data of the pixels in the range of the metal sheet of the other combination of two components and the second threshold value boundary indicates an abnormality. a camera (81, 82) configured to capture an image of the metal sheet whose abnormality is to be detected; and an image processing section configured to divide luminance data of pixels in a range of the metal sheet from the image captured by the camera (81, 82) into three components of an R value, a G value, and a B value, characterized in that the image processing section is configured to detect a rolling abnormality on the surface of the metal sheet on the basis of a relationship between the luminance data of two components among the luminance data of the three components; and in that the abnormality detecting apparatus further comprises a database configured to store information about a first threshold value boundary determined beforehand for dividing a normal value and an abnormal value from each other from a distribution produced by plotting, from a collected plurality of image data representing normal and abnormal data, on a two-dimensional graph of a combination of two components, luminance data of one component of an R value, a G value, and a B value of the combination of two components and luminance data of another component of the combination of two components, and a second threshold value boundary determined beforehand for dividing a normal value and an abnormal value from each other from a distribution produced by plotting, from the collected plurality of image data, on a two-dimensional graph of another combination of two components, luminance data of one component of the other combination of two components and luminance data of another component of the other combination of two components, a first threshold value boundary determined beforehand for dividing a normal value and an abnormal value from each other from a distribution produced by plotting, from a collected plurality of image data representing normal and abnormal data, on a two-dimensional graph of a combination of two components, luminance data of one component of an R value, a G value, and a B value of the combination of two components and luminance data of another component of the combination of two components, and a second threshold value boundary determined beforehand for dividing a normal value and an abnormal value from each other from a distribution produced by plotting, from the collected plurality of image data, on a two-dimensional graph of another combination of two components, luminance data of one component of the other combination of two components and luminance data of another component of the other combination of two components, wherein the image processing section is configured to detect a rolling abnormality if a detected result obtained from the luminance data of the pixels in the range of the metal sheet of the combination of two components from among the R value, the G value, and the B value and the first threshold value boundary indicates an abnormality and if a detected result obtained from the luminance data of the pixels in the range of the metal sheet of the other combination of two components and the second threshold value boundary indicates an abnormality.