To provide a decryption key transmission device, a computer program, a decryption key transmission system and a decryption key transmission method which can control the browsing of data by controlling the transmission of a decryption key for decrypting encrypted data.

E-mail is encrypted by a personal computer 1a, and the encrypted e-mail is transmitted from the personal computer 1a to a personal computer 1b via an e-mail server apparatus 2a. The personal computer 1a transmits a browsing key for decrypting the encrypted e-mail and a message ID to the e-mail server apparatus 2a, and the e-mail server apparatus 2a stores the message ID and the browsing key of the received encrypted e-mail by making them associated with each other. The e-mail server apparatus 2a receives the message ID transmitted from the personal computer 1b, and when the browsing key associated with the received message ID is stored, transmits the browsing key to the personal computer 1b. The personal computer 1b decrypts the received encrypted e-mail using the browsing key received from the e-mail server apparatus 2a.