The present invention relates to a process for selectively producing a cellulose ether derivative with a very high reaction efficiency of an organohalide compound. There is provided a process for producing a cellulose ether derivative such as carboxymethyl cellulose and hydroxyethyl cellulose in which a low-crystalline powdery cellulose having a crystallinity of 50% or less is reacted with an organohalide compound such as ethylene chlorohydrin, a monohaloacetic acid and a salt of the monohaloacetic acid in the presence of a base.