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.