An image-forming unit 100 in a complex device A forms a correction-use image onto a sheet in accordance with data, in such a manner that the correction-use image thus formed partly on and partly off the sheet and includes edges of the sheet, the date being preliminarily determined in accordance with a size of the sheet. An image-detecting section 520 reads that portion of the correction-use image which is transferred on an image-transfer belt 7 as the correction-use image is formed by the image-forming unit 100. A correction value is calculated in accordance with data thus obtained by the image-detecting section 520. A control section then causes a data storing section to store the correction value. The control section reads out the correction value when forming an image in the complex device A, and forms the image, performing correction of the image, using the correction value for correcting an image-forming condition regarding how the image is formed on the sheet. The image-forming condition, such as a position and a copy-scale, can be adjusted, even if using an unadjusted reading apparatus.