When a cryptographic communicating part 208 of the communication support server 20 exchanges information with the information processing units 14, if the term of validity of a first key stored in a cryptographic key storing part 200 and corresponding to the identification information of the information processing unit 14 does not expire, the cryptographic communicating part 208 performs the cryptographic communication with the information processing unit 14 using the first key, without performing a process of authenticating the information processing units 14. When the term of validity of the first key expires or the first key corresponding to the identification information of the information processing units 14 is not stored, the key sharing part 202 shares the first key with the information processing units 14, and the cryptographic communicating part 208 performs the cryptographic communication with the information processing units 14 using a newly shared first key.