Sheet stacking apparatus and image forming apparatus

An image forming apparatus comprising:

• an image forming part which forms an image on a sheet;
• and
• a sheet stacking apparatus which stacks a sheet on which an image is formed by the sheet forming part, wherein said sheet stacking apparatus capable of using a plurality of sheet stack trays by switching with each other, wherein said sheet stacking apparatus comprises:
• a plurality of sheet stack trays each of which moves to keep an uppermost surface of sheets stacked on the sheet stack tray in a constant position from a sheet discharge outlet when sheets are discharged on each of said plurality of sheet stack trays,
• wherein said plurality of sheet stack trays includes:
• a first sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said first sheet stack tray have been stacked thereon, which is prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked;
• and
• a second sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said second sheet stack tray have been stacked thereon, which is not prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked,
• wherein in a case where the number of sheets stacked on the first sheet stack tray reaches a predetermined number of sheets to be stacked which does not prevent stacking of sheets onto said another sheet stack trays until the number of sheets stacked on said another sheet stack trays reach the maximum number of sheets stacked, stacking of sheets on the first sheet stack tray is temporarily prohibited to stack sheets by switching the present sheet stack tray to a sheet stack tray other than the first sheet stack tray reaching the predetermined number of sheets.