In a heat print device for a stamp unit, a dummy stamp unit is mounted on the heat print device to tentatively thermally print a character array of normal characters on the heat sensitive sheet of a dummy stamp unit before a stamp unit is mounted on the heat print device to perform perforation printing of a character array of mirror characters of the normal characters on the print face portion of a stamp unit. If the character array formed on the heat sensitive sheet of the dummy stamp is satisfactory with respect to the character array, the character size, and the arrangement of the character array, the stamp unit is mounted on the heat printing device to perform the perforation print of the character array of the mirror characters on the print face portion of the stamp unit.