1. An image forming apparatus that performs image formation on a recording sheet according to image data, comprising: a selecting unit configured to select image data to be used for the image formation; a preview display control unit configured to control displaying an image to be formed on the recording sheet in the same size as when printed thereon, as a real-size preview image, before executing the image formation on the recording sheet according to the image data selected by said selecting unit; and an operation key control unit configured to control an operation key to be operable or not to be operable, wherein when the operation key has been operated, said preview display control unit is configured to control displaying the real-size preview image; wherein said operation key control unit is configured to control the operation key to be operable, when predetermined processing has been executed to the image data selected by said selecting unit.