The forgery of document are among the offences against public confidence. In forgery of the document, the real situation in the document that is suitable for legal consequences is hidden. In order for forgery to be accepted as a crime, the document, which is the material subject of the crime, must have the power to deceive third parties. The power of deception, which can be expressed as the deception ability, is the material element of the forgery of document. In the concrete case, the act is not a crime in case of deficiency. When forgery is considered as fraudulent behavior, the deception ability is a certain degree of quality of fraud. Whether the document has the deception ability must be evaluated by the judicial authority. The fact that the person to whom the document is given does not understand that there has been a forgery does not prove the existence of her deception ability. The evaluation should be based on objective criterion. Forgery should not be understood without special investigation.