The invention provides a polyvinyl acetal having a degree of acetalization of from 45 to 80 mol %, which is obtained through acetalization of a polyvinyl alcohol that contains from 1 to 15 mol % of -olefin units and has a 1,2-glycol bond content of from 1 to 3 mol %, a degree of polymerization of from 100 to 2000 and a degree of hydrolysis of from 80.0 to 99.99 mol %. The polyvinyl acetal has good waterproofness and good compatibility with plasticizer. Having the advantages, it is favorable for interlayer films for laminated glass, binders for ceramic forming, binders for ink or paint, and coating liquids for thermally-developable photographic materials.