PURPOSE: To enhance synthetic reaction efficiency of chemical substance, by causing pressure drop in liquid flowing in a pipe and thereby controlling the inside of the pipe at high pressure and also precisely controlling the retention time of reaction in the pipe.

CONSTITUTION: Liquid contg. chemical substance having reactivity therewith is introduced into the liquid contg. chemical substance flowing in a pipe and pressure drop is caused in the liquid flowing in the pipe and the inside of the pipe is controlled at high pressure and thereby reaction is efficiently performed. Simultaneously the retention time in the pipe is precisely controlled and a reaction product is obtained by precisely controlling the reaction of mutual chemical substances. The liquids in the pipe are preferably allowed to react with each other under heating and high-temp. control and also mixing in the reaction tube is made efficient by connecting a pipe having different diameter to the pipe and also high pressure is preferably caused in the reaction tube.