A component in a processing chamber of a substrate processing apparatus, where a temperature may be accurately measured by using a temperature measuring apparatus using an interference of a low-coherence light, even when a front surface and a rear surface are not parallel due to abrasion, or the like. A focus ring used in a vacuum atmosphere and of which a temperature is measured includes an abrasive surface exposed to an abrasive atmosphere according to plasma, a nonabrasive surface not exposed to the abrasive atmosphere, a thin-walled portion including a top surface and a bottom surface that are parallel to each other, and a coating member coating the top surface of the thin-walled portion, wherein a mirror-like finishing is performed on each of the top and bottom surfaces of the thin-walled portion.