An image forming apparatus includes an image forming device, a trim mark adder, a duplex alignment mark adder and a hardware processor. The image forming device is capable of forming an image on both sides of a sheet based on an image data. The trim mark adder adds trim mark information to the image data so that a trim mark is formed at a trimming position of the sheet. The duplex alignment mark adder adds duplex alignment mark information to the image data so that a duplex alignment mark is formed in a margin area between an edge of the sheet and the trimming position on both sides of the sheet. The hardware processor performs a duplex alignment of the image data based on a result of scanning the duplex alignment mark.