PROBLEM TO BE SOLVED: To provide an amino acid-containing liquid obtainable by quickly treating and recycling a food processing waste liquid by effectively utilizing plant cell or fiber contained in it.SOLUTION: In this method of manufacturing an amino acid-containing liquid, an acetic acid bacterium and a lactic acid bacterium are added to a food processing waste liquid containing plant cells or fiber, and acetic acid fermentation and lactic acid fermentation are carried out alternately by intermittent aeration, then the amino acid-containing liquid is obtained by solid-liquid separation. It is preferable, before the input of bacteria, to inject a vapor-liquid two-phase flow in the food processing waste liquid to break or forced-oxidize the cell walls and cell membranes of the plant cells in suspended matter, and thus the food processing waste liquid is quickly recycled. The amino acid-containing liquid can be used as a fertilizer, a harmful insect repellent or their raw material, or the like.
