Provided are a chat system for realizing good communication between users, a control method for a chat device and a chat server, and an information storage medium. Upon receiving a message, a message transfer unit (11a) transfers the message to a game terminal of a transferring destination, and a stop restriction unit (11c) transmits an instruction (restricting instruction) to restrict a chat function stop processing on the game terminal which has transmitted the message, thereby restricting stopping of the chat function on the game terminal. A stop restricting time calculation unit (11d) calculates a period (stop restricting period T) for restricting the chat function stop processing in accordance with the character count (p) of the received message and the judgment by a keyword presence/absence judgment unit (11e), and a timer unit (11b) specifies a stop restricting time end time. Upon arrival of the stop restricting time end time, the stop restriction unit (11c) transmits an instruction (restriction release instruction) to release the restriction on the chat function stop processing.