1. A communication terminal comprising a rewritable non-volatile memory and a rewritable volatile memory, the communication terminal further comprising: transfer status transition means for limiting operation of software which uses the rewritable volatile memory and securing an area required for storing an update file in the rewritable volatile memory before receiving update software as the update file from a software management server which manages the update software; transfer request transmission means for requesting transfer of the update file to the software management server after the transfer status transition means secures the area required for storing the update file in the rewritable volatile memory; update file reception means for receiving the update file from the software management server, and storing the update file in the rewritable volatile memory; and software rewriting means for rewriting software, which is stored in the rewritable non-volatile memory and is directly executed, with the update software stored in the rewritable volatile memory as the update file, after completion of storing the update file to the rewritable volatile memory by the update file reception means.