A mobile terminal having a communication function. The apparatus includes: a nonvolatile memory including a memory area storing a main-unit program including an update engine for updating software and an alternative unit area for a unit area in the memory area; a working memory; and control means for accessing both of the memories. The control means downloads differential data between a version of the main-unit program and a version of a new main-unit program, expands the update engine into the working memory area, executes the update engine, updates the main-unit program for each unit area by the differential data, completes the update processing if the update has been successful, and performs the unit-area update processing on the alternative unit area and replaces the defective unit area by the alternative unit area if a defect of a unit area has been detected at the time of the update.