According to a portable medium (IC card) of this invention, a memory area is divided into a system program area and a user program area. A text transmitted between the IC card and a host system connected thereto includes a command text and a response text. The command or response text includes a flag indicating that the text is written in the user program area or the system program area. The memory area has a conversion table for a function code and the start address of a program corresponding to the function code. The conversion table is looked up using the given function code as a parameter, thereby obtaining the start address of the corresponding program. The memory area has a correspondence table between a newly added function code and the start address of an added function program. The added function program is selectively executed.