PURPOSE: To provide a screw designing method for a twin-screw extruder capable of designing a proper screw form at every material and product.

CONSTITUTION: A preprocessor creates a mesh data by dividing a resin passage in a barrel with a hexahedron cell element three-dimensionally. A heat flow calculating section analyzes a numerical value by various boundary conditions and resin properties based on an actual phenomenon based on the mesh data and disperses it, and analyzes a heat flow for fixing physical quantity histories of temperature, shear rate, and viscosity. A postprocessor displays a trajectory in the area to be analyzed, based on the flow velocity dispersed by the numerical analysis and displays each physical quantity graphically on the mesh data. By using a value to be outputted by the heat flow analysis, any of a resin temperature distribution, a shear rate distribution, and a shearing stress distribution after passage of a melt resin through a full-filling part in an extruder is made to be an index. By using the index, a dimension of a screw is designed.