A terminal device 4 transmits a certificate issue request including a communication ID thereof and a sub ID to a certificate issuing device 7 via a NW1 (a first network). The certificate issuing device 7 inquires of a communication ID (identifier) checking device 5 whether or not the communication ID included in the certificate issue request is in use or not and inquires of a communication ID/sub ID checking device 6 whether or not the communication ID and the sub ID are associated with each other. If both the check results are OK, the certificate issuing device 7 generates a certificate including the ID of the certificate issuing device 7, the communication ID, the sub ID and a validity period and transmits the certificate to the terminal device 4. In this way, a certificate with a short validity period can be issued only based on the access to the NW1 using the communication ID and the sub ID.