To provide a collaboration system and its chat system wherein a chat can be conducted by designating clearly and simply a position of an object to be noticed in a working image.[Constitution] A plurality of terminal devices 100 and a chat server 300 supporting a chat are provided. By managing a display position, on a display screen, of a chat window for performing a chat in the terminal devices 100 and the chat server 300, display positions of chat windows are synchronized between the terminal devices 100 that conduct a chat. The chat window is displayed correspondingly to a given element displayed on the display screen.