A security device of this invention includes a nonvolatile storage unit 22 for storing a validity check unit including a counter updated every time signature function means 30 is called up, a volatile storage unit 24 for reading and storing a counter array out of an external nonvolatile storage unit storing the counter array, in which the counter array is obtained by coupling a hash value generated for each signature key with a signature number counter for counting the number of signatures performed by use of the signature key, and a hash function unit 28 for reading the counter array out of the volatile storage unit 24, generating the hash value, and transferring the hash value to the validity check unit for a validity check.