To provide a stable composition containing a hydrolyzate of an N-vinylformamide polymer, in which a change in the structural units of the polymer after production is suppressed.

In the composition containing a hydrolyzate of an N-vinylformamide polymer, the hydrolyzate of the N-vinylformamide polymer contains vinylformamide units, vinylamine (salt) units and 6-membered cyclic amidine units, has a cationization rate defined by following expression (1) of ≥20 mol%, and contains water whose upper limit content is 15% and formic acid or its salt whose upper limit content is 15%. Expression (1): {(vinylamine units)+(6-membered cyclic amidine units)}/{(vinylamine (salt) units)+(vinylformamide units)+2×(6-membered cyclic amidine units)}.