the data is ensured by having a certificate authority issue a certificate the certificate issued by a certificate authority is the data for proving that the public key for digital signature analysis is authentic next the client creates a request step s 2 to be more specific the client creates the request 33 1 as the client request 33 shown in fig3 for example the request 33 1 can be created by clickably selecting each icon indicative of a request for product purchase or content download further the client creates a signature step s 3 to be more specific the client creates two pieces of information the request 33 1 and the data 33 2 as the client request 33 shown in fig3 for example the client generates a hash value when encrypting these two pieces of information the request 33 1 and the data 33 2 with a signature 33 3 that is a private key the generated signature is the encrypted signature information for ensuring the validity of each digital document at this moment the client sends the client request 33 shown in fig3 to the server 32 step s 4 then the client receives the server response 1 34 shown in fig3 from the server 32 step s 5 next the client executes verification by use of the certificate step s.