A message sending device comprising: a storage unit that stores characteristic phrases that are character strings registered by a user or defined based on a term frequency-inverse document frequency, TF-IDF, calculated based on a message, the storage unit storing each of the characteristic phrases in association with respective identification information for identifying the characteristic phrase; an obtaining unit configured to obtain an operation log recording how the user has operated the message sending device , to determine the identification information of a characteristic phrase associated with an input operation during creation of a message based on the operation log, and to obtain, from the operation log and corresponding to the identification information, a respective operation characteristic of an input operation during creation of the message, the operation characteristic including one of: an operation characteristic of a key input during inputting of a characteristic phrase; an operation characteristic of an operation performed on a screen; and an attitude-wise operation characteristic; an addition unit configured to add, to the message, the identification information of the characteristic phrase and the corresponding operation characteristic obtained by the obtaining unit , the message sending device being for use with a message receiving device which comprises a collating unit configured to collate an operation characteristic that has been added to a message received by the message receiving device with a registered operation characteristic, and a warning unit configured to issue a warning depending on a collation result of the collating unit , the message sending device further comprising: a sending unit configured to send the message, to which the operation characteristic has been added by the addition unit , to the message receiving device . a storage unit that stores characteristic phrases that are character strings registered by a user or defined based on a term frequency-inverse document frequency, TF-IDF, calculated based on a message, the storage unit storing each of the characteristic phrases in association with respective identification information for identifying the characteristic phrase; an obtaining unit configured to obtain an operation log recording how the user has operated the message sending device , to determine the identification information of a characteristic phrase associated with an input operation during creation of a message based on the operation log, and to obtain, from the operation log and corresponding to the identification information, a respective operation characteristic of an input operation during creation of the message, the operation characteristic including one of: an operation characteristic of a key input during inputting of a characteristic phrase; an operation characteristic of an operation performed on a screen; and an attitude-wise operation characteristic; an operation characteristic of a key input during inputting of a characteristic phrase; an operation characteristic of an operation performed on a screen; and an attitude-wise operation characteristic; an addition unit configured to add, to the message, the identification information of the characteristic phrase and the corresponding operation characteristic obtained by the obtaining unit , the message sending device being for use with a message receiving device which comprises a collating unit configured to collate an operation characteristic that has been added to a message received by the message receiving device with a registered operation characteristic, and a warning unit configured to issue a warning depending on a collation result of the collating unit , the message sending device further comprising: a sending unit configured to send the message, to which the operation characteristic has been added by the addition unit , to the message receiving device . <EOS>