During a sheet-through operation, for a first original, a shading operation is performed to acquire white data and a light-quantity measuring operation is performed to acquire a reference value. For subsequent originals, only the light-quantity measuring operation is performed to acquire a subsequent light quantity. If the reference value and the subsequent light quantity differ greatly, the shading operation is performed at this time point to acquire new white data. The sheet-through operation is not stopped when the shading operation is not performed and the sheet-through operation is stopped when the shading operation is performed.