Means for enabling plating on sites of complex configuration, etching for fine complex pattern, etc. through reduction of the viscosity resistance brought about by walls of fine liquid channel of microreactor. In particular, a microreactor comprising a liquid inlet, a fine liquid channel and a liquid discharge zone characterized in that the liquid channel is formed of a magnetic barrier of band ferromagnet so that a magnetic liquid introduced through the inlet undergoes at least one operation of chemical reaction, mixing, extraction and absorption in the liquid channel. Further, there is provided means for plating or etching performed by causing a plating solution or an etching solution to flow through the liquid channel.