Method for producing water-soluble cellulose ether having low degree of polymerization and method for producing film coating composition comprising same

Provided are a method for producing a water-soluble cellulose ether having a low degree of polymerization and enhanced whiteness， and the like. The method comprises the steps of: bringing a pulp powder having a multiplication product of less than 0.004 mm 2 which is obtained by multiplying a number-average fiber length by a number-average fiber width， each measured with a Kajaani fiber length analyzer， into contact with an alkali metal hydroxide to obtain an alkali cellulose; reacting the alkali cellulose with an etherifying agent to obtain a crude water-soluble cellulose ether having a high degree of polymerization; purifying the crude water-soluble cellulose ether; drying the purified water-soluble cellulose ether; grinding the dried water-soluble cellulose ether into a water-soluble cellulose ether powder; and depolymerizing the water-soluble cellulose ether powder to obtain the water-soluble cellulose ether having a low degree of polymerization.