A message processing device includes a memory and a processor coupled to the memory. The processor is configured to receive a transmission instruction from a first information processing device. The transmission instruction includes a first message, first authentication information to be used for first authentication of authenticating a first user, and first identification information for identifying a first storage area. The processor is configured to identify the first storage area by using the first identification information. The processor is configured to store the first message in the identified first storage area. The processor is configured to perform the first authentication by using the first authentication information. The processor is configured to store, when the first authentication is successfully completed, information indicating authenticity in the first storage area in association with the first message.
