PURPOSE: To form a cold water-soluble film which easily dissolves in cold water and which has superior properties such as long-term stability, acid resistance and alkali resistance, by using an acetalized polyvinyl alcohol formed with the restriction of the degree of acetalization and the length of the alkyl chain of an aldehyde used.

CONSTITUTION: A cold water-soluble film comprises a mixture of a component (A) and a component (B), wherein said component (B) is 30W90wt.% (A) is a polyvinyl alcohol derivative having a basic structure consisting of a vinyl alcohol unit, a vinyl acetate unit and an acetalized vinyl alcohol unit wherein the viscosity average degree of polymerization of the vinyl monomer unit is 200W500; the vinyl alcohol unit is 70W90mol.%; the vinyl acetate unit is 0W2mol.%; the degree of acetalization is 10W30mol.%; and the average length of the alkyl chain in R (alkyl group) of the acetalized vinyl alcohol unit of formula I is 0.34W1.70, and (B) is the same as the component (A) except having a viscosity-average degree of polymerization of 550W2,500.