In a sheet stacking apparatus, a sheet stacking unit stacks sheets conveyed by a sheet conveyance device. Three sheet extraction portions allow to selectively extract the sheets stacked on the sheet stacking unit from one of three directions including the downstream side of the sheet conveyance direction and two directions perpendicular to the sheet conveyance direction. A plurality of first detection sensors are provided in at least two of the three sheet extraction portions to detect an object that enters the sheet stacking unit. A second detection sensor detects the moving direction of the sheets stacked on the sheet stacking unit. A control device stops the operation of the sheet conveyance device when the first detection sensor detects the object, and disables the detection operation of the first detection sensor corresponding to the moving direction of the sheets when the second detection sensor detects the moving direction of the sheets.