Abstract: A method of marking a number on a work. The number is read out by a data read-out means, the read-out data is electrically transmitted and returned in a secure manner, and a predetermined number is securely marked. The structure is made such that the chassis number is read out again at a time of reading out the chassis number from a pass card P on which the chassis number is displayed by a bar code reader (data read-out means) R and that the read-out data is transmitted to a sequence device (an arithmetic control apparatus) S so as to judge whether good or no good. Further, the data transmitted to a marking system (a marking means) M from the sequence device S is returned to the sequence device S from the marking system M so as to judge whether good or no good. Then, in the case that the received data is no good, the marking operation is stopped.