Mobile terminal and software update method

A mobile terminal having a communication function, comprising:

• 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 the nonvolatile memory and the working memory,
• wherein 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 of the memory area into the working memory area, executes the update engine in the working memory area, performs update processing of 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.