An ammonia selective reduction-type NOx catalyst (48) is interposed in an exhaust passage of an engine (2) provided in a series-type hybrid electric vehicle (1), and a urea water injector (52) is disposed in the exhaust passage to supply a urea water into exhaust gas existing upstream of the ammonia selective reduction-type NOx catalyst (48). The engine (2) is started and stopped in accordance with the storage state of a battery (8), and the urea water injector (52) is controlled in accordance with the operating state of the engine (2). When the engine (2) is to be stopped, an adsorption increasing operation for increasing the amount of ammonia adsorbed by the ammonia selective reduction-type NOx catalyst (48) is executed over a predetermined period before the engine (2) is stopped.