1. An image forming apparatus comprising: an image reading portion that scans a document image and generates image data; an image processing portion that processes image data generated by the image reading portion; a storage portion including a first storage portion with low vibration resistance whose data transfer speed is reduced due to vibrations, and a second storage portion with higher vibration resistance than the first storage portion, that stores the image data processed by the image processing portion; a storage selector portion that selects one of the first storage portion and the second storage portion as a storage location of the processed image data, based on the strength of mechanical vibrations during an operation of the image forming apparatus, and a data size of the processed image data; a image forming portion that forms an image based on the image data stored in the storage portion; and a control portion that causes the image data processed by the image processing portion to be stored in one of the first storage portion and the second storage portion selected by the storage selector portion, and in addition, controls the image forming portion to read the image data from one of the first storage portion and the second storage portion storing the image data, and form an image.