To easily use different types of sheet trays, for example, the ones for standard-sized sheets and nonstandard-sized sheets in compliance with the requests of users.

A sheet accommodating device, installed in an image forming device, for accommodating the sheets P within the sheet tray 1 is detachably fitted with the different type (e.g. 1a, 1b) of the sheet tray 1. Additionally, the device is provided with a marker 3 (e.g. 3a, 3b), mounted on the side of the sheet tray 1, for distinguishing the kind of the sheet tray 1, a discriminating means 4, mounted on the side of a tray receiving part 2, for discriminating the type of the sheet tray 1 (1a, 1b) on the basis of the marker (3a, 3b) on the side of sheet tray 1 (1a, 1b). A sheet processing executing means 6 is also equipped for executing different kinds of sheet processing according to the type of the sheet tray 1 discriminated by the means 4. This image forming device is assembled with above sheet accommodating device.