In a pair of preceding and following cylinders whose exhaust and intake strokes overlap each other, intake air supplied to the preceding cylinder (2A, 2D) is supercharged by a turbocharger (23) to produce combustion at a “lean” air-fuel ratio in the preceding cylinder (2A, 2D), and burned gas discharged from the preceding cylinder (2A, 2D) is introduced into the following cylinder (2B, 2C) through an intercylinder gas channel (22). Combustion in the following cylinder (2B, 2C) is made at an air-fuel ratio equal to or smaller than the stoichiometric air-fuel ratio by supplying fuel to the burned gas of a “lean” air-fuel ratio introduced from the preceding cylinder (2A, 2D), and gas discharged from the following cylinder (2B, 2C) is led to an exhaust passage (20) provided with a three-way catalyst (30).