To uniformly mix a liq. or solid additive continuously and without thermal modification and to eliminate special cleaning of the inside of a pipeline when the additive is switched by a method wherein the additive is injected into the pipeline and the additive is continuously dispersed on the surface of synthetic resin particles being pneumatically conveyed in the pipeline.

A method for adding a solid additive to a particulate synthetic resin pref. consists of a method wherein particles flow from the bottom to the top in a vertical pneumatic conveying pipeline 5 and a funnel 13 is provided on the top end of a nozzle 14 for addition provided at angle θ in the flow direction and the solid additive is sucked into the pipeline 5 and is mixed by making the solid additive falling down into the inlet by means of a quantitative feeder. In the case of a liq. additive, it is pref. that the nozzle 14 for addition is inserted into the central part of the pneumatic conveying pipeline 5 and the liq. additive is injected into a nozzle 14 by using a pump. It is possible thereby that the additive is instantaneously dispersed by turbulent and dispersing effect of a flow and is sticked uniformly and continuously on the surface of the resin particles. In addition, the resin particles move by repeating collision on the inner wall of the pipeline and exhibit a self-cleaning effect for the inner wall.