To sign an electronic mail to be transmitted with an electronic signature even when a private key is not held in a user terminal that receives an instruction for transmitting the electronic mail.

An electronic mail communication device includes a key acquisition unit 107 which obtains the private key of a transmission user according to an instruction for signing the electronic mail with the electronic signature from a key management server 300 that holds the private key of a user, an encryption/decryption processing unit 108 which signs the electronic mail with the electronic signature by using the private key of the transmission user obtained by the key acquisition unit 107, and a communication control unit 102 which transmits the mail that is signed with the electronic signature by the encryption/decryption processing unit 108.

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