An abnormality detecting apparatus that detects a rolling abnormality on a surface of a metal sheet to be rolled by a rolling mill, the abnormality detecting apparatus comprising: a camera (81, 82) configured to capture images of the metal sheet that is an object to be detected for the rolling abnormality during rolling of the metal sheet; a luminance standard value setting section configured to extract the metal sheet from at least one image captured by the camera and obtain a luminance standard value by which the strip is determined to have no abnormality from luminance data of pixels within a range of the metal sheet extracted from the at least one image; and an image processing section configured to extract the metal sheet from a latest image among the captured images and detect the rolling abnormality on a basis of a luminance difference between the luminance data of the pixels within the range of the metal sheet extracted from the latest image and the luminance standard value; wherein the luminance standard value setting section is configured to divide the luminance data extracted from the at least one image into three components of an R value, a G value, and a B value, and obtain, as the luminance standard values, a standard R value, a standard G value, and a standard B value, wherein the image processing section is configured to divide the luminance data of the pixels within the range of the metal sheet extracted from the latest image into an R value, a G value, and a B value, subtract the standard R value from the R value, the standard G value from the G value, and the standard B value from the B value to obtain luminance differences extracted from the latest image, and detect the rolling abnormality on a basis of the relationship between the luminance differences of two components among the respective luminance differences extracted from the latest image, characterized in that the abnormality detecting apparatus further comprises a database in which a threshold boundary separating a normal value from an abnormal value is obtained in advance from distribution obtained by plotting, a plurality of times, a luminance difference between a component value and a standard value of one component against the luminance difference between a component value and a standard value of another component on a two-dimensional graph of two components among the R value, the G value, and the B value in images in which a metal sheet is reflected from a plurality of pieces of image data of normality and abnormality collected in advance, wherein the image processing section is configured to detect the rolling abnormality on a basis of the luminance difference of the one component extracted from the latest image, the luminance difference of the other component extracted from the latest image, and the threshold boundary. a camera (81, 82) configured to capture images of the metal sheet that is an object to be detected for the rolling abnormality during rolling of the metal sheet; a luminance standard value setting section configured to extract the metal sheet from at least one image captured by the camera and obtain a luminance standard value by which the strip is determined to have no abnormality from luminance data of pixels within a range of the metal sheet extracted from the at least one image; and an image processing section configured to extract the metal sheet from a latest image among the captured images and detect the rolling abnormality on a basis of a luminance difference between the luminance data of the pixels within the range of the metal sheet extracted from the latest image and the luminance standard value; wherein the luminance standard value setting section is configured to divide the luminance data extracted from the at least one image into three components of an R value, a G value, and a B value, and obtain, as the luminance standard values, a standard R value, a standard G value, and a standard B value, wherein the image processing section is configured to divide the luminance data of the pixels within the range of the metal sheet extracted from the latest image into an R value, a G value, and a B value, subtract the standard R value from the R value, the standard G value from the G value, and the standard B value from the B value to obtain luminance differences extracted from the latest image, and detect the rolling abnormality on a basis of the relationship between the luminance differences of two components among the respective luminance differences extracted from the latest image, characterized in that the abnormality detecting apparatus further comprises a database in which a threshold boundary separating a normal value from an abnormal value is obtained in advance from distribution obtained by plotting, a plurality of times, a luminance difference between a component value and a standard value of one component against the luminance difference between a component value and a standard value of another component on a two-dimensional graph of two components among the R value, the G value, and the B value in images in which a metal sheet is reflected from a plurality of pieces of image data of normality and abnormality collected in advance, wherein the image processing section is configured to detect the rolling abnormality on a basis of the luminance difference of the one component extracted from the latest image, the luminance difference of the other component extracted from the latest image, and the threshold boundary.