The terminal device 100 of an information sharing system 500 exchanges information mutually with other terminal devices 200 to 400 that are connected via a network 600. A receiving unit 103 receives adoptability information showing update data for updating common data that are shared among a group and adoptability information showing the adoptability of update data. An inputting unit 101 receives an input of the update data and adoptability information from a user. A sending unit 104 sends the inputted update data and adoptability information to another terminal device. A synthesizing unit 105 generates the updated common data by synthesizing the update data with the common data. A judging unit 109 judges adoptability of the update data based on the adoptability information and has the synthesizing unit 105 synthesize only the update data that is judged as adoptable.