The governing rheological property for extrusion drawing in film-casting process is proposed in this study. The experiment of film-casting process using the high-pressure process low-density polyethylene (LDPE) was performed. The non-isothermal viscoelastic simulation of the film casting experiment was also carried out to explain the experimental results. Film width reduction phenomenon in an air gap, so-called neck-in behavior, was investigated by using the simulation of the LDPE and the model fluids exhibiting specific viscoelasticity. The neck-in phenomenon was also examined using theoretical model based on force balance and deformation type of a film. As a result, the neck-in normalized by the air gap was in good correlation with the ratio of planar to uniaxial elongational viscosity rather than the strain hardening nature of uniaxial elongational viscosity.