To provide a photographic polyester base excellent in dimensional stability even at a high temperature due to heat development, having little difference in the rate of dimensional change in all directions and showing dimen sional change either in extension or in shrinkage with the low rate of the change, hardly causing anisotropy even at a high temperature, hardly causing breakage because of high mechanical strength and excellent also in planeness and to provide a method for producing the base and a silver salt photothermographic dry imaging material.

In the method for producing the photographic polyester base by extruding a molten polyester on a cooling drum to form an unstretched sheet and subjecting this sheet to biaxial stretching, heat fixation and cooling, the biaxial stretching is carried out by simultaneous biaxial stretching and heat treatment is carried out after the cooling.