Image formation apparatus that can ensure provision of sheets for image formation according to external image data

1. Field of the Invention An image formation apparatus to ensure provision of sheets used in a host mode process and a guest mode process without increasing the number of sheet cassettes to be mounted at a sheet feed unit and to prevent tedious task due to increase in the sheet feed frequency is provided. In the image formation apparatus, the main CPU identifies the accommodated number of sheets n in the sheet cassette every time one copy mode image formation process ends. This value n is compared with a set value of the number of sheets N read out via a sub CPU. When value n is equal to or less than value N, the copy mode image formation process is interrupted until the sheet cassette is replenished with sheets while waiting for input of image data associated with the guest mode image formation process. When image data associated with the guest mode image formation process is input from an external device during this period, the guest mode image formation process is executed.