An image forming device is provided with a sheet feeding unit capable of selectively feeding recording sheets of a plurality of sizes, a print unit configured to print out images represented by print data on page-by-page basis, the print data including information of designated size of the recording sheets. On condition that running out of the recording sheets of the designated size in the sheet feeding unit is detected during printing out of the images, the sheet feeding unit starts feeding the recording sheets of a substitute size which is different from the designated size and the print unit continues printing out of images represented by remaining part of the print data on the recording sheet of the substitute size. A storing unit stores the remaining part of the print data after printing, but not part of the print data printed on the recording sheets of designated size.