A facsimile device comprising: an operation part for accepting a setting relating to cryptographic communication; a storage part for storing therein a same common key as in a reception-side facsimile device; a transmission data generation part for generating message data by subjecting image data to an encoding process for facsimile communication; an encryption key generation part for generating an encryption key by selecting, as a cryptograph-generation applied signal(s), one or plural kinds of signals from among signals received from the reception-side facsimile device and notifying a number or function to the reception-side facsimile device and moreover performing a first arithmetic operation with use of the common key and a signal value of the cryptograph-generation applied signal; an encryption part for generating encrypted data by performing encryption of the message data with use of the encryption key generated by the encryption key generation part; and a communication part for performing transmission and reception of signals with the reception-side facsimile device before start of message transmission and moreover transmitting the encrypted data and decryption information including information indicative of a kind of the cryptograph-generation applied signal to the reception-side facsimile device, wherein the operation part displays a crypto-communication setting screen and accepts a setting of a level in cryptographic communication based on operation in the crypto-communication setting screen, the transmission data generation part generates message data including an error detecting code, given a setting in the operation part that cryptographic communication is executed at a predetermined middle level, the encryption key generation part generates an encryption key by performing the first arithmetic operation with use of the common key and the signal value of the cryptograph-generation applied signal, the communication part transmits, to the reception-side facsimile device, the decryption information including information indicative of the kind of the cryptograph-generation applied signal, given a setting in the operation part that cryptographic communication of a level higher than the predetermined middle level is executed, the encryption key generation part generates an encryption key by performing a second arithmetic operation with use of the common key, the signal value of the cryptograph-generation applied signal and the value of the error detecting code within the message data, and the communication part transmits, to the reception-side facsimile device, the decryption information including information indicative of the kind of the cryptograph-generation applied signal and information indicative of the value of the error detecting code used for generation of the encryption key. an operation part for accepting a setting relating to cryptographic communication; a storage part for storing therein a same common key as in a reception-side facsimile device; a transmission data generation part for generating message data by subjecting image data to an encoding process for facsimile communication; an encryption key generation part for generating an encryption key by selecting, as a cryptograph-generation applied signal(s), one or plural kinds of signals from among signals received from the reception-side facsimile device and notifying a number or function to the reception-side facsimile device and moreover performing a first arithmetic operation with use of the common key and a signal value of the cryptograph-generation applied signal; an encryption part for generating encrypted data by performing encryption of the message data with use of the encryption key generated by the encryption key generation part; and a communication part for performing transmission and reception of signals with the reception-side facsimile device before start of message transmission and moreover transmitting the encrypted data and decryption information including information indicative of a kind of the cryptograph-generation applied signal to the reception-side facsimile device, wherein the operation part displays a crypto-communication setting screen and accepts a setting of a level in cryptographic communication based on operation in the crypto-communication setting screen, the transmission data generation part generates message data including an error detecting code, given a setting in the operation part that cryptographic communication is executed at a predetermined middle level, the encryption key generation part generates an encryption key by performing the first arithmetic operation with use of the common key and the signal value of the cryptograph-generation applied signal, the communication part transmits, to the reception-side facsimile device, the decryption information including information indicative of the kind of the cryptograph-generation applied signal, given a setting in the operation part that cryptographic communication of a level higher than the predetermined middle level is executed, the encryption key generation part generates an encryption key by performing a second arithmetic operation with use of the common key, the signal value of the cryptograph-generation applied signal and the value of the error detecting code within the message data, and the communication part transmits, to the reception-side facsimile device, the decryption information including information indicative of the kind of the cryptograph-generation applied signal and information indicative of the value of the error detecting code used for generation of the encryption key.