This invention relates to a sheet loading device for causing sheets put out one after another from a sheet output apparatus such as a laser beam printer or a copying apparatus onto a sheet receiving member to be loaded in orderly, mutually superposed relationship without being deviated from one another. The sheet loading device is provided with a sheet receiving member for supporting thereon discharged sheets in mutually superposed relationship, a sheet loading positioning member, gathering device for frictionally contacting the upper surface of the sheets supported on the sheet receiving member and gathering the sheets in an oblique direction, and a support member free to move and position and supporting the gathering device and the positioning member, and is characterized in that the support member is moved in accordance with the size of the sheets put out onto the sheet receiving member to thereby position the gathering device and the positioning member mounted on the support member at positions corresponding to and matching the size of the sheets. The sheet loading device is also characterized in that during sheet loading operation, the sheet receiving member cannot be placed out of the device and the sheet receiving member cannot be inserted into the device unless it is properly prepared.