A device and a method of providing a to-do list of a user are provided. The device includes a controller configured to collect behavior information about behavior between the user and another user, the behavior being performed by using the device, generate a to-do list of the user based on the collected behavior information, and determine an unperformed task not performed by the user from among at least one task in the to-do list by using log information about an operation of the device, and an output unit configured to output notification information in a dialogue style, along with a notification reason for notifying the determined unperformed task.