A movement device having an endless belt with a working portion defined in a substantially horizontal working plane and a reserve portion defined in a reserve plane optionally coinciding with said working plane; a first guide wheel linearly moveable in said working plane and guiding the endless belt; a second guide wheel linearly moveable in said reserve plane and guiding the endless belt; two deflector members for deflecting the endless belt from the working plane to the reserve plane; drive mechanism for driving the endless belt; and movement mechanism for synchronously moving the two guide wheels in such a manner that the sum of the distances between each of the guide wheels and the deflector members is constant. The length of the working portion of the endless belt is capable of varying continuously and the quantity of articles that can be placed on the working portion of the belt can vary accordingly.