An original reading device corrects line noise when performing original image reading by a sheet-through method, even if line noise appears due to a piece of debris simultaneously influencing all color sensors. Background plates (33) have different densities and a background plate switching motor (37) switches between the background plates in order. Before the original reaches a reading position, an image of each background plate is read to generate correction data. Image data is generated by reading an image of the original when it has reached the reading position. A noise address is detected using the correction data and/or image data. The correction data is used to calculate an influence value indicating a level of influence of a dirtiness source inferred to exist at an area corresponding to the noise address. In the image data, a line noise portion indicated by the noise address is corrected using the influence value.