A diffusion reaction method includes: joining a plurality of reaction liquids relating to a reaction to form a multilayer flow; sandwiching from both sides of the multilayer flow in the depthwise direction using sandwiching liquid so that the multilayer flow is contracted and thinned; and flowing the multilayer flow through a reaction channel to cause a diffusive mixing between laminar flows so as to cause the reaction. The method enables to allow reacting diffusive mixing a plurality of reaction liquids instantly in a reaction channel, is suitable for any kinds of reaction product. Further, for example, when forming fine particles in a diffusion reaction, the method can prevent logging of the reaction channel by the fine particles and an adhesion of the fine particles to the wall of the reaction channel.