To provide a heat storage device in which decomposition of ammonia reacting with a chemical heat storage material can be prevented.

A heat storage device 10 includes: a reactor 11 which is arranged on the upstream side of a diesel oxidation catalyst (DOC) 4 in an exhaust passage 3; a medium tank 13 which is connected with the reactor 11 via a conduit 12 and stores ammonia; and a cooling fan 14 which sends cooling air for cooling the reactor 11 into the reactor 11. In the reactor 11, a compact (chemical heat storage material) containing CaCl₂, an air flow passage for causing the cooling air to flow and an ammonia flow passage for causing the ammonia (NH₃) which chemically reacts with CaCl₂ to flow are disposed. In order to improve the cooling efficiency of the reactor 11, it is desirable to use compressed air as the cooling air.