An image forming apparatus comprising: an operation portion that inputs various operating instruction information and sets a plurality of processing operations; 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 (15,16) 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 stores a threshold size information in which a threshold size is set for each of the plurality of processing operations categorized according to a strength of mechanical vibrations to maintain processing speed of the image forming apparatus at the data transfer speed of the first storage portion during each processing operation, and selects one of the first storage portion and the second storage portion as a storage location of the processed image data, based on the threshold size information and a data size of the processed image data; an image forming portion that forms an image based on the processed image data stored in the storage portion (15,16); 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 processed image data from one of the first storage portion and the second storage portion storing the processed image data, and form an image, wherein the storage selector portion specifies a threshold size corresponding to the processing operation set by the operation portion, based on the threshold size information, and selects the first storage portion when the data size of the processed image data is less than the specified threshold size, and selects the second storage portion when the data size of the processed image data is not less than the specified threshold size. an operation portion that inputs various operating instruction information and sets a plurality of processing operations; 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 (15,16) 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 stores a threshold size information in which a threshold size is set for each of the plurality of processing operations categorized according to a strength of mechanical vibrations to maintain processing speed of the image forming apparatus at the data transfer speed of the first storage portion during each processing operation, and selects one of the first storage portion and the second storage portion as a storage location of the processed image data, based on the threshold size information and a data size of the processed image data; an image forming portion that forms an image based on the processed image data stored in the storage portion (15,16); 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 processed image data from one of the first storage portion and the second storage portion storing the processed image data, and form an image, wherein the storage selector portion specifies a threshold size corresponding to the processing operation set by the operation portion, based on the threshold size information, and selects the first storage portion when the data size of the processed image data is less than the specified threshold size, and selects the second storage portion when the data size of the processed image data is not less than the specified threshold size.