Thermogravimetric-Mass Spectrometry (TG-MS) is effective for the analysis of synthetic rubber because it is possible to correlate and obtain on the weight loss and qualitative information on the generated gas when heating is applied. On the other hand, since this method does not use chromatogram separation, many rubber pyrolysis products (mainly hydrocarbons) make it difficult to analyze trace components. Therefore, mass separation by high-resolution TOFMS JMS-T2000GC was performed and useful results were obtained. While this report presents the results of the EI method, MSTips 344 reports the results of the Electron Capture Ionization (ECI) method.