The present invention has an object of the present invention to provide a polyvinyl acetal resin for heat-developable photosensitive materials as well as a heat-developable photosensitive material while solving such problems as coating solution pot life, coloration of heat-developable photosensitive material, fog, poor gradation, insufficient sensitivity and poor undeveloped film storability and making it possible for the materials to acquire good image characteristics.The present invention is constituted of a polyvinyl acetal resin for heat-developable photosensitive materials which is a polyvinyl acetal resin synthesized by the acetalization reaction between a polyvinyl alcohol and an aldehyde and which comprises having a degree of polymerization of 200 to 3,000, a residual acetyl group content of 0 to 25 mole percent and a residual hydroxyl group content of 17 to 35 mole percent, as calculated while regarding one acetal group as two acetalized hydroxyl groups, a water content of not more than 2.5% by weight and a residual aldehyde content of not more than 10 ppm and is free of any antioxidant.