Polymer degradation in the extrusion process decreases quality and productivity. For this reason, it is necessary to prevent polymer degradation. In the extrusion process, polymer degradation is caused by oxidation. It depends on the processing temperature and the amount of dissolved oxygen in the molten polymer. Therefore, a quantitative analysis of these factors is required. As for the degradation characteristics of the material used in this study, temperature and oxygen concentration dependency of the oxidation rate could be quantitatively characterized with an apparatus to evaluate polymer degradation which utilized chemiluminescence generated by an oxidation reaction. Moreover an online measuring apparatus to analyze dissolved gas in the extruded molten polymer was developed. With this apparatus, the volume ratio of dissolved gases (N-2, O-2 etc.) to the extruded molten polymer could be quantitatively analyzed and the quality of the extruded molten polymer evaluated. With this apparatus, dissolved nitrogen (an index of entrained air) was analyzed with a full-flight screw and a barrier screw. Furthermore, observation of cross sectional views in the screw channel obtained from the cooling experiment under the operating conditions was carried out for the full-flight screw and the barrier screw. With the full-flight screw, break up phenomenon (collapse of solid polymer) occurred in the screw channel and the amount of nitrogen increased. With the barrier screw, the amount of nitrogen decreased because of prevention of the break up phenomenon in the screw channel. Consequently, it is shown that the use of the barrier screw is suitable for oxygen reduction in the molten polymer, which is a factor in causing polymer degradation.