Engine exhaust emission purifier

A nitrogen oxides trapping catalytic converter (20) is provided downstream of a three-way catalytic converter (18) of an exhaust passage (31) of an engine (1). An engine control unit (21) operates the engine (1) at a stoichiometric air-fuel ratio until the three-way catalyst becomes active so as to provide a minute amount of oxygen contained in the burnt gas of the fuel mixture to the zeolite catalyst which traps NOx in the presence of oxygen. After the three-way catalyst is activated, the engine (1) is operated at a lean air-fuel ratio. Due to this air-fuel ratio control, oxygen supply to the zeolite catalyst is ensured regardless of the activation state of the three-way catalyst.