To provide a method for controlling oxygen density in production of a cellulose derivative, in particular a cellulose ether.

This method for controlling oxygen density of a gaseous phase part in a closed reaction system comprises controlling following three action time factors in the method for producing the cellulose ether by reacting cellulose pulp with an alkali metal hydroxide and an etherifying agent; (a) inert gas introduction time, (b) mixed gas introduction time consisting of oxygen and an inert gas, and (c) oxygen density measuring time by shutdown of the gas introduction.