1. An apparatus for validating a valuable document including a plurality of value markings arranged to form a first code word, said apparatus comprising; means for reading said first code word to determine if said first code word is valid, means, responsive to said determination, for erasing in each of a plurality of erasing steps, one or more of said value markings, after each erasing step the erased and remaining unerased value markings forming another code word, a control and decision unit for determining if said sequence of other code words resulting from said erasing steps corresponds to a predetermined sequence of code words, and for producing a signal if said determination is affirmative, and a reading head and an erasing head for reading and erasing said value markings, a first output of said control and decision unit being connected to a control input of said reading head, a second output of said control and decision unit being connected to a control input of said erasing head, and wherein the outputs of said reading head are led to an input of said control and decision unit.