Hydrolysis in high temperature and high pressure water was studied using semi-batch reactor with quick injection system at preexisting stable reaction conditions. This is useful for accurate analysis of the initial reaction stage of soild material decomposition. In this study, the system is applied to investigate the production behavior of amino acid udring initial stage of hydrothermal decomposition of proteins in fish-derived wastes. The apparatus is also employed to study a process and temperature for effective production of amino acids. As a result, simple amino acids such as Gly, Ala, Ser and Asp were dominantly present in the products. Maximum yield of total amino acids (229 mg/g-dry entrails) was obtained at 300 ℃ and a sampling time of 2.5 min.