To obtain a mail letter or standard-size letter for mailing a thin plate-like member or particularly an optical and/or magnetic information medium to be described on a part of an envelope without enclosing an invoice or an instruction manual.

A mounting sheet has an opening for containing a thin plate-like member and is formed of a flexible member having a thickness substantially equal to that of the member. The mounting sheet is adhered to a document describing area 13-1 of a main letter with a temporary adhesive. After an invoice or a description of the member is described on the area 13-1 formed on an inner surface of the letter, the member is engaged with the opening of the mounting sheet. Since the mounting sheet is coated with the temporary adhesive, the sheet can be easily released. The invoice or the description of the member is described under the mounting sheet, and hence much information can be incorporated in the letter.