To obtain a vinyl alcohol-based polymer capable of imparting a composition excellent in thermal stability, waterproof, a barrier property against gas or steam, low temperature leaving stability in an aqueous solution and biodegradability by specifying the content of ethylenic units or the like.

This polymer has an ethylenic unit content of 2-19 mol%, a polymerization degree of 200-2,000, a saponification degree of 8099.99% and a total content of carboxyl groups and lactone rings of 0.02-0.4%. The total content of carboxyl groups and lactone rings preferably meets the formula (wherein P is a viscosity average polymerization degree of the vinyl alcohol- based polymer), it is especially preferable that the content of 1,2-glycol bonds is 1.2-2%, the molar fraction of central hydroxyl groups in a hydroxyl group triplet shown by triad designation to vinyl alcohol units is 65-98% and the melting point is 160-230°C.