A method of providing a reminder based on a chat room performed by a chat room based reminder apparatus connected to at least one user terminal, the method including: bringing a message thread corresponding to a chat room related to a user terminal and a to-do item set of at least one to-do item related to the message thread and including a to-do receiver, a to-do provider, to-do contents, a to-do status, and a deadline; generating a reminder set including at least one reminder to-do item satisfying a reminder condition in the to-do item set; generating a reminder message for each of the at least one reminder to-do item; and detecting a time zone of the to-do receiver and delaying to add the reminder message to the message thread to a receivable time of the to-do receiver.