A unit-to-unit data exchange system preventing an unfair exchange without increasing communications cost or a load on a server. A unit (11a) exchanges data with a unit (11b). An exchange completion data keeping device keeps exchange completion data that is necessary for each unit (11a and 11b) to reproduce the obtained data after the data exchange is completed. The exchange completion data keeping device transmits the exchange completion data obtained from the unit (11b) to the unit (11a) and transmits the exchange completion data obtained from the unit (11a) to the unit (11b) only when the unit (11a) and the unit (11b) have successfully received data of interest.