A sheet post-process apparatus comprises left and right lower face support members which support left and right lower faces in a transport direction of a sheet retained by a waiting tray in a waiting mode, distal end lower face support members which support a distal end lower face of the sheet transmitted to the waiting tray, a distal end upper face retainer member which retains a distal end upper face of the sheet transported to the waiting tray, sensor means for sensing that a sheet has been held on the waiting tray, and a drive motor which, based on a sense signal of the sensor means, drives the distal end upper face retainer member in a location for retaining a distal end upper face of the sheet transported to the waiting tray.