EGA-MS (Evolved Gas Analysis-Mass Spectrometry) instrument has been constructed with skimmer interface that is effective to measure adsorptive gaseous species and useful to infrared image furnace. The skimmer interface is based on the principle of a jet separator. Thus objective gaseous species with higher mass have been enriched because of the difference of diffusion velocity from helium with light mass. The apparatus has successfully analyzed carbon substances in materials by sol-gel processing.