A message string output system which can prevent users from feeling uncomfortable due to repeated input of a particular string by another user is provided. A judging unit (36) judges whether or not a message string which is input by an input unit (30) contains a specific string. An input status information updating unit (38) updates, based on a judgment result by the judging unit (36), input status information stored in an input status information storage (46) in corresponding to user identification information corresponding to the message string. An output form determining unit (43) determines an output form of a specific string part contained in the message string based on the input status information stored in correspondence with the user identification information corresponding to the message string.