The purpose of the present invention is to provide a vinyl acetal polymer which has a low solution viscosity and excellent handleability, while exhibiting excellent solubility in alcohol-based solvents and the like and excellent bondability to inorganic materials such as glass. The present invention is a vinyl acetal polymer which is obtained by acetalizing a vinyl alcohol polymer, and which is characterized in that: the vinyl alcohol polymer contains a monomer unit having a group represented by formula (1) and satisfies formula (I); and the degree of acetalization is from 45% by mole to 80% by mole (inclusive). It is preferable that the vinyl alcohol polymer also satisfies formulae (II) and (III). 370 <= P S <= 6,000 (I) 200 <= P <= 4,000 (II) 0.1 <= S <= 10 (III) P: Viscosity-average degree of polymerization S: Rate of content of above-mentioned monomer unit (mol%)