A disclosed image forming apparatus includes: an image data input unit inputting image data; an image data storage unit storing the image data; a storage method change unit changing a storage method of the image data; an image data output unit outputting the image data; an available capacity detection unit detecting available capacity of the image data storage unit; an available capacity judgment unit judging whether the available capacity is not more than a predetermined value; and a storage method selection unit prompting selection of a storage method performed by the image data storage unit. When the available capacity is judged to be not more than the predetermined value, the storage method selection unit prompts the selection of a storage method, the storage method change unit changes the storage method of the image data, and the image data output unit outputs image data.