Provided are: a freeze-dried composition which contains (a) a monovalent metal salt of alginic acid and (b) a salt selected from among monovalent metal salts and ammonium salts； and a method for producing a freeze-dried composition of a monovalent metal salt of alginic acid, which comprises a step wherein an aqueous solution, in which at least the above-described component (a) and component (b) are dissolved, is frozen and primarily dried and then, if desired, is dried secondarily so that the moisture content thereof becomes 3% by mass or less. The present invention is a freeze-dried composition of a monovalent metal salt of alginic acid, which is suppressed in decrease of viscosity over time.
