NOVELTY - A polyester resin inputted is molten and mixed into the biaxial extruder in the cylinder. The number of rotations of screw is controlled in the cylinder and satisfies a specific relation, molten and extruded, to obtain polyester resin.    USE - Production of polyester resin used for protection sheet for solar cell module (all claimed).    ADVANTAGE - The polyester resin is efficiently and stably manufactured with suppressed vent up in the melting-kneading time.    DETAILED DESCRIPTION - A polyester resin inputted is molten and mixed into the biaxial extruder in the cylinder. The number of rotations of screw is controlled in the cylinder and satisfies the relation of formulae (1)-(5). The relation of formula (1) is when 0 less than or equal to Q less than or equal to 0.5x Q0, then (Y'-Y'min)/Q less than or equal to (Y'max-Y'min)/(0.5x Q0), where Y' is unit extrusion amount (Q/N(kg/hr/rpm)) per 1 rotation of screw, Y'min and Y'max are Y'min ((kg/hr/rpm)) which is 9.01x 10-7x D3, Y'max (kg/hr/rpm) is 6.93x 10-6x D3, D is diameter (mm) of screw, Q is extrusion amount of melted polyester resin per hour in the driving, Q0 is extrusion amount in the normal driving without increase and decrease change or decrease and increase change, and N is number (rpm) of rotations of screw. The relation of formula (2) is when 0.5x Q0 less than Q, then Y' less than or equal to Y'max. The relation of formula (3) is Y' greater than or equal to Y'min. The relation of formula (4) is 0.6x Q0 less than or equal to Qmax less than or equal to 1.2x Q0, where Qmax is maximum extrusion amount and minimum extrusion amount in the increase and decrease change or decrease and increase change. The relation of formula (5) is 0.15x Q0 less than or equal to Qmin less than or equal to 0.4x Q0, where Qmin is maximum extrusion amount and minimum extrusion amount in the increase and decrease change or decrease and increase change. The extrusion amount of Y' is changed continuously during melting and extruding. The extrusion amount of Qmin to Qmax is increased or Qmax to Qmin is decreased during melting and extruding, to obtain polyester resin.