It is an object of the invention to provide an engine system using a small-sized heat exchanger having a high efficiency and a stable performance for heating and vaporizing a hydrogen medium supplied to a reactor vessel. The engine system includes an engine having an intake pipe for taking in air for combustion thereinto and an exhaust pipe for exhausting exhaust gas therefrom, a reactor vessel with a catalyst supported therein for decomposing a hydrogen medium which contains hydrogen in molecules thereof and of which the hydrogen can be taken out by a chemical reaction, a hydrogen medium tank storing the hydrogen medium, a high-pressure pump for pressurizing and transporting the hydrogen medium, a first heat exchanger for exchanging heat with the hydrogen medium in a liquid state, a second heat exchanger for exchanging heat with the hydrogen medium in a gas state, and a back pressure valve for depressurizing and vaporizing the hydrogen medium, disposed in a hydrogen medium flow path between the first heat exchanger and the second heat exchanger, wherein the reactor vessel is disposed in the exhaust pipe so that the reactor vessel can exchange heat with the exhaust pipe, and wherein the high-pressure pump, the first heat exchanger, and the second heat exchanger are arranged in the hydrogen medium flow path for transporting the hydrogen medium from the hydrogen medium tank to the reactor vessel in this order from an upstream side in the hydrogen medium flow path.