To facilitate cutting, folding of a sheet in the case of making an envelope by a general consumer himself by opening an opening 1 for filling in a postal code number on an upper right corner of a slender rectangular plate having specific lateral and longitudinal sizes, and forming the envelope using the sheet of A4 of a standard size as it is.

The guide plate for manufacturing an envelope is formed of a slender rectangular plate material having a size within an allowable size of a first-class standard-size mail, a longitudinal side of 230 mm and a lateral side of 107 mm. The lateral size is desired to be 107 mm decided as a maximum value of a size standard of a commerciallyprinted postcard so that the plate can be used as a guide when the postcard is formed. As its raw material, a stainless steel sheet is desired to be used by considering corrosion resistance or the like. Its plate thickness t is about 0.5 mm so that riding of a cutting edge scarcely occurs in the case of using the plate as a cutter guide and the sheet is easily bendable. An opening 1 for filling-in seven postal code numbers of a longitudinal side of 8.0×5.7 mm is opened at an interval of 1.2 mm at the upper right corner of the plate material.