To provide a reactor enabling a chemical reaction in a volume from 10 to 100 μm for continuously treating 100-1,000 specimens.

This is a reactor for a chemical reaction having a capillary reacting part filled with a carrier liquid not miscible with water, wherein the reactor consists of (1) a specimen injector injecting aqueous specimens into the capillary at every predetermined interval, (2) a system for pumping a transportation liquid and aqueous specimen in the capillary, (3) a thermostat for controlling the temperature of the capillary reactor, and (4) a detector.