An image forming apparatus which enables easy management of proofs in a printing process. An image forming apparatus 299 is provided with a reader unit 50, a transfer charger 220, color sensors 274-1 to 274-4, a printer controlling unit 21. The reader unit 50 inputs image data for forming an image on a sheet having an RFID tag 701. The transfer charger 220 generates a transfer image for forming the image on the sheet based on the inputted image data. The color sensors 274-1 to 274-4 measure image formation conditions of the transfer image. The printer controlling unit 21 writes the measured image formation conditions into the RFID tag 701.