A sheet post-process apparatus according to one example of the present invention includes a movable stack tray for stacking a sheet that is elevated in a vertical direction; a movable tray position detection unit that detects a vertical position of the movable stack tray while a request command for post-processing of the sheet is not received; an upper surface sensor that senses either an upper surface of the movable stack tray or an upper surface of the sheet stacked on the movable stack tray when a first position of the movable stack tray detected by the movable tray position detection unit is lower than or equal to a predetermined first height; and a movable tray moving unit that moves the movable stack tray upward when either the upper surface of the movable stack tray or the upper surface of the sheet stacked on the movable stack tray is not sensed by the upper surface sensor.