A heat storage device 10 comprises a reactor 11, arranged in an exhaust system, having a formed body 15 chemically reacting with ammonia and generating heat; and an ammonia reservoir 13, connected to the reactor 11, for storing ammonia. The ammonia reservoir 13 incorporates therein an absorbent physically absorbing ammonia. The ammonia reservoir 13 has a heat-exchanging structure and is temperature controlled by heat exchange with an external heating medium.