To efficiently execute processings by controlling detection of dust and a processing time and maintaining compatibility with productivity.

The image processing apparatus includes an image processing means for receiving image data to be outputted from a reading means for reading an original document in a sheet-through method, and applying predetermined processing to the received image data. In the image processing apparatus, if the time t1 between an original document transferred to an original reading position and a subsequent original document to be transferred for reading is enough, the processing time t2 for dust detection and dust correction is ensured much; and if the time t1 is short or the other processings increase, the processing time t2 for the dust detection processing and dust correction processing is reduced, thereby ensuring reading productivity. The processing time t2 is determined by managing the time t1 with a system controller. The processing time of each of the dust detection and dust correction may be independently set.