We study X-ray characteristics of shocked ambient gas of the Galactic core-collapse supernova remnant Cas A. Using 1 Msec observation with Chandra X-ray Observatory, we identify thermal emissions from the shocked ambient gas along the outer boundary of the remnant. Our results show that Cas A is expanding into a circumstellar wind with a wind density n ~ 1 $cm^{-3}$ at the current outer radius of the remnant (~ 3 pc). We suggest that the progenitor star of Cas A, which exploded as a Type~IIb SN, had an initial mass ~16 Msun, and have lost ~10 Msun as a RSG wind. We discuss the implications of our results for the mass loss of massive stars and the resulting supernova type.