The invention provides a portable telephone set wherein a countermeasure can be taken against a bug of software by a user without the necessity for voluntary recovery by a maker. The portable telephone set has a hardware configuration which allows a user of the portable telephone set to perform an operation of downloading a modification portion of software from the outside such as a network and rewriting the software with the same. In this instance, the rewriting is performed not for the entire memory (ROM) space of the portable telephone set, but the software is divided and disposed into different blocks of the memory (ROM) space and the rewriting is performed in a unit of a block.