Cassiopeia A is a nearby young supernova remnant that provides a unique laboratory for the study of core-collapse supernova explosions1. Cassiopeia A is known to be a type IIb supernova from the optical spectrum of its light echo2, but the immediate progenitor of the supernova remains uncertain3. Here, we report results of near-infrared, high-resolution spectroscopic observations of Cassiopeia A, where we detected the pristine circumstellar material of the supernova progenitor. Our observations revealed a strong emission line of iron (Fe) from a circumstellar clump that has not yet been processed by the supernova shock wave. A comprehensive analysis of the observed spectra, together with a Hubble Space Telescope image, indicates that the majority of Fe in this unprocessed circumstellar material is in the gas phase, not depleted onto dust grains as in the general interstellar medium4. This result is consistent with a theoretical model5,6 of dust condensation in material that is heavily enriched with carbon？nitrogen？oxygen cycle nuclear reaction products, supporting the idea that the clump originated near the helium core of the progenitor7,8. It has recently been found that type IIb supernovae can result from the explosion of a blue supergiant with a thin hydrogen envelope9,10,11, and our results support such a scenario for Cassiopeia A.