To show the mail characters on a display screen in the easy-to-see sizes by deciding whether the mail characters are smaller than the allowable sizes or not and then changing the mail character sizes into those of an allowable range if the mail character sizes are larger than the allowable ones.

The known data which are necessary for conversion of the character codes into the display dot patterns as plural fonts of different character sizes are stored in a secondary storage 105 together with the transmitting and receiving mails. Then the mail data are read out of a database contained in the storage 105, and a central arithmetic processing unit 102 stores the font size of the mail data in a storage 103. This font size is compared with a set font size to decide whether the characters are smaller the allowable sizes or not. If the characters are larger than the allowable sizes, these character sizes are changed into those of an allowable range and displayed.