To allow an individual person to easily pay a charge when the radio telephone set is used for a public digital portable telephone system when pre- paid charge information indicates it that the pre-paid charge is insufficient for a speech.

When a radio section in use by a portable telephone set 1 is switched into a public digital radio section 131, the section 131 reads a pre-paid charge information 121 and pre-paid charge information from a card inserted to a card processing section 170 to decide whether or not the remaining charge is enough for a public network speech and stores the result to a storage section 120. When it is decided that the pre-paid charge information 121 indicates that the remaining charge is enough for the public network speech, a dialing processing 124 is called and the pre-paid charge information is displayed on a display section 160 and the public digital radio section 131 is used for dialing. When it is decided that the pre-paid charge information 121 indicates that the remaining charge is not enough for the public network speech, a voice processing section 150 raises a warning tone denoting that the pre-paid charge is insufficient and a display denoting that the pre-paid charge is insufficient is made on the display section 160.