Abstract: The main purpose of this technique is to newly provide a technique whereby it becomes possible to shorten the time for anaerobic fermentation by reducing the concentration of oxygen contained in a raw material by a method other than an inert gas replacement method that requires the introduction of a facility. This technique provides a method for producing a fermented food or beverage, including a saccharide oxidase action step for allowing a saccharide oxidase to act on a portion or the whole of a saccharide in a raw material, and an anaerobic fermentation step for performing anaerobic fermentation. This technique also provides an oxygen concentration reducing agent for anaerobic fermentation use which contains a saccharide oxidase. This technique further provides an anaerobic fermentation method which includes a saccharide oxidase action step for allowing a saccharide oxidase to act on a portion or the whole of a saccharide in a raw material.