A microreaction system consisting of a micromixer and a capillary reaction component was developed for the continuous production of amino acid polymers. A silicon glass based chip-type micromixer tethered with a three dimensional microchannel network was fabricated by a combination of maskless technique, dry-etching, and sequential anode bonding. The microreactor was constructed by connecting the micromixer with a microcapillary reaction component and a syringe pump. The resulting microreactor showed excellent controllability of molecular weight distribution of polyamino acid synthesis. The microreactor also produced amino acid copolymers with a more highly controlled molecular weight distribution than that of a batchwise reaction system operating for two months. These features demonstrate that the new microreactor is extremely suitable for the production of polyamino acids.