A software update system comprising: an information terminal; and a software download server connected to the information terminal via a network, the software download server storing update software; the software update system characterized in that the software update system further comprises a security unit communicable with the information terminal via a near field radio network, the software download server creates a time-limited encryption key, computes a hash value of each of files included in the update software to create a hash table in which hash values of the files are listed, and encrypts the hash table using the encryption key, the security unit obtains the encrypted hash table and the encryption key from the software download server, and the information terminal obtains the encrypted hash table from the security unit as a first table, obtains the encryption key from the security unit, determines whether or not a time limit of the encryption key is valid, obtains an encrypted hash table as a second table from the software download server if the time limit is determined to be valid, the encrypted hash table as the second table being created by the same software download server as the encrypted hash table as the first table, decrypts the first and second tables using the encryption key, compares the first and second tables after decryption to determine whether or not the download server is appropriate, initiates download of the update software from the software download server if it is determined both tables are identical to each other, computes hash values of files included in the downloaded update software as the download is completed, compares the computed hash values with the hash values in either the first or second table after the decryption, and permits activation of the downloaded update software if both hash values are identical to each other. an information terminal; and a software download server connected to the information terminal via a network, the software download server storing update software; the software update system characterized in that the software update system further comprises a security unit communicable with the information terminal via a near field radio network, the software download server creates a time-limited encryption key, computes a hash value of each of files included in the update software to create a hash table in which hash values of the files are listed, and encrypts the hash table using the encryption key, the security unit obtains the encrypted hash table and the encryption key from the software download server, and the information terminal obtains the encrypted hash table from the security unit as a first table, obtains the encryption key from the security unit, determines whether or not a time limit of the encryption key is valid, obtains an encrypted hash table as a second table from the software download server if the time limit is determined to be valid, the encrypted hash table as the second table being created by the same software download server as the encrypted hash table as the first table, decrypts the first and second tables using the encryption key, compares the first and second tables after decryption to determine whether or not the download server is appropriate, initiates download of the update software from the software download server if it is determined both tables are identical to each other, computes hash values of files included in the downloaded update software as the download is completed, compares the computed hash values with the hash values in either the first or second table after the decryption, and permits activation of the downloaded update software if both hash values are identical to each other.