PURPOSE: To improve mixing of the adding agent and mixture for each cylinder by a method wherein the injection nozzles of the adding agent added to the suction of the engine are provided on an intake manifold.

CONSTITUTION: The nozzles 1 for injecting the adding agent are provided on the branches of the intake manifold 2 for respective cylinders. The tip end part of the outer cylinder 4 of the nozzle 1 is provided on the periphery thereof with a discharging port 12 in order to remain a reservoir 11 for the adding liquid of a normal pressure and the pressurized adding liquid and the adding liquid of the normal pressure is discharged as the droplet of the liquid while the pressurized adding liquid is discharged in an atomized condition by pressurized air coming from the upper part thereof.