A sheet stacking apparatus includes a first and a second guide plate having a guide surface inclined to a perpendicular plane to take an open state having a gap of a first interval in a direction crossing the inclination direction and a closed state of a second interval narrower than the first interval, a sheet take-in portion standing by under the first and second guide plates to support sheets dropped from the guide surfaces with the leading edge thereof in a conveying direction directed downward in a standing position, and a controller configured to permit the first and second guide plates to enter the closed state and guide another sheets conveyed following the sheets supported internally by the sheet take-in portion by the guide surfaces and then to enter the open state and drop the another sheets from the gaps onto the preceding sheets.