There is provided a polyoxyalkylene-modified vinyl acetal polymer which has polyoxyalkylene groups represented by general formula (I) in side chain, in which a viscosity-average polymerization degree P is 150 to 5000, an acetalization degree is 10 to 85 mol % and a polyoxyalkylene modification rate S is 0.1 to 10 mol %. Such a polymer has a feature that a film or sheet produced therefrom has improved film strength and flexibility, as well as a feature that a resin composition therefrom is free from phase separation. wherein R1 represents a hydrogen atom or a methyl group, R2 represents a hydrogen atom or an alkyl group having 1 to 8 carbon atoms, and 1≦m≦10 and 3≦n≦20.