There is provided a vinyl alcohol-based polymer having an olefin in side chain, comprising 0.001 to 10 mol % of a structural unit represented by Formula (1) based on the total amount of structural units, wherein the total carbon number of X, R1, R2, R3 and R4 is 2 or more. The vinyl alcohol-based polymer having an olefin in side chain has excellent storage stability, good solubility in water or an organic solvent even after thermal treatment, and excellent reactivity to high energy beam. In Formula (1), X represents an optionally substituted divalent aliphatic hydrocarbon group, an optionally substituted divalent alicyclic hydrocarbon group, an optionally substituted divalent aromatic hydrocarbon group, or a group consisting of two or more of these groups which are linked via at least one bond selected from the group consisting of an amide bond, an ester bond, an ether bond, and a sulfide bond; R1, R2, R3 and R4, independently of each other, represents a hydrogen atom, an optionally substituted aliphatic hydrocarbon group, an optionally substituted alicyclic hydrocarbon group, or an optionally substituted aromatic hydrocarbon group; and X, R1, R2, R3 and R4 can be arbitrarily combined to form a ring structure).