An ammonia selective reduction-type NOx catalyst (48) is interposed in an exhaust passage of an engine (2) installed in a series-type hybrid electric vehicle (1), and a urea-water injector (62) for supplying urea water in the upstream side of the ammonia selective reduction-type NOx catalyst (48) is placed in the exhaust passage. The engine (2) started in response to a storage state of a battery (8) is operated in a first operation mode, and urea water supply is suspended, until exhaust temperature reaches predetermined temperature (Ta). Once the exhaust temperature reaches the predetermined temperature (Ta), the engine (2) is operated in a second operation mode, and urea water is supplied from the urea-water injector (62). While the engine (2) is operated in the first operation mode, NOx emissions from the engine (2) are more decreased than in the second operation mode.