A novel apparatus for rapid introduction of solid sample into a semibatch reactor at preexisting stable high-temperature and high-pressure water conditions is developed. This unique system avoids heating effects on the feed associated with a batch process and difficulties with pumping a solid feed in a continuous process. This is useful for accurate analysis of the initial reaction stage of solid material decomposition. In this study, the apparatus is applied to investigate the behavior of the amino acid composition during the initial stages of the hydrothermal decomposition of proteins in fish-derived wastes. In relation to our previous studies, the apparatus is also employed to study a process for the effective production of amino acids. The effect of temperature on the yield of amino acids at the short reaction time of 2.5 min using this reactor is also investigated.