An imaging apparatus for forming the image on a sheet, wherein the imaging apparatus includes a sheet-feeding path for conveying the sheet, a detecting section positioned along the sheet-feeding path, which detects a leading edge, a trailing edge and a side edge of the sheet while the sheet is being conveyed along the sheet-feeding path, a controller for registering dimensions, a memory for storing data of dimensions wherein when a auto-measurement mode is set in the controller, the controller controls the detecting section so as to detect the leading edge, the trailing edge and the side edge of the sheet while the sheet is being conveyed along the sheet-feeding path and obtains a length and a width of the sheet, and then the controller registers the length and the width of the sheet to be stored in the memory.