An image processing apparatus which comprises a scanner for reading an image of a document sheet set on a document feeder and a printer for printing the read image on a print sheet stored in a specified sheet feeding tray of a plurality of sheet feeding trays (201, 202, 203), the image processing apparatus comprising: detecting means (SW01, SW02, SW03, SW04) for detecting a document size of the document sheet set on the document feeder; obtaining means for obtaining a sheet size of the print sheet stored in the specified sheet feeding tray; and execution means for executing a copy job to print the read image of the document sheet on the print sheet stored in the specified sheet feeding tray, characterized in that the image processing apparatus further comprises control means for controlling, in a case where the sheet size obtained by the obtaining means is not a predetermined size, the scanner to read an image of the document sheet in a reading region based on at least the document size of the document sheet detected by the detecting means (SW01, SW02, SW03, SW04), for controlling, in a case where the sheet size obtained by the obtaining means is the predetermined size, and the document size detected by the detecting means (SW01, SW02, SW03, SW04) is not same as a document size to be detected by the detecting means (SW01, SW02, SW03, SW04) when the print sheet is set on the document feeder, the scanner to read an image of the document sheet in a reading region based on at least the document size detected by the detecting means (SW01, SW02, SW03, SW04), and for controlling, in a case where the sheet size obtained by the obtaining means is the predetermined size, and the document size detected by the detecting means (SW01, SW02, SW03, SW04) is same as a document size to be detected by the detecting means (SW01, SW02, SW03, SW04) when the print sheet is set on the document feeder, the scanner to read an image of the document sheet in a reading region based on at least the sheet size obtained by the obtaining means. detecting means (SW01, SW02, SW03, SW04) for detecting a document size of the document sheet set on the document feeder; obtaining means for obtaining a sheet size of the print sheet stored in the specified sheet feeding tray; and execution means for executing a copy job to print the read image of the document sheet on the print sheet stored in the specified sheet feeding tray, characterized in that the image processing apparatus further comprises control means for controlling, in a case where the sheet size obtained by the obtaining means is not a predetermined size, the scanner to read an image of the document sheet in a reading region based on at least the document size of the document sheet detected by the detecting means (SW01, SW02, SW03, SW04), for controlling, in a case where the sheet size obtained by the obtaining means is the predetermined size, and the document size detected by the detecting means (SW01, SW02, SW03, SW04) is not same as a document size to be detected by the detecting means (SW01, SW02, SW03, SW04) when the print sheet is set on the document feeder, the scanner to read an image of the document sheet in a reading region based on at least the document size detected by the detecting means (SW01, SW02, SW03, SW04), and for controlling, in a case where the sheet size obtained by the obtaining means is the predetermined size, and the document size detected by the detecting means (SW01, SW02, SW03, SW04) is same as a document size to be detected by the detecting means (SW01, SW02, SW03, SW04) when the print sheet is set on the document feeder, the scanner to read an image of the document sheet in a reading region based on at least the sheet size obtained by the obtaining means.