An acetalized polyvinyl alcohol having a molecular weight of 10,000 to 1,000,000 represented by the formula ##STR1## wherein: R.sup.1 represents a residue of an aldehyde or a ketone the residue having no photosenitivity to ultraviolet rays; R.sup.2 represents a hydrogen atom, which may partially be substituted with an acetyl group; R.sup.3 represents naught or a monomeric unit copolymerizable with vinyl acetate; and l, m, n, are integers indicating polymerization degrees, has excellent characteristics such as high sensitivity, high resolving power and excellent dry etching resistance and is suitable as a negative-type resist in ionizing radiation lithography.