A center ring interposed between a vacuum chamber and a vacuum pump, comprises: a ring main body including a first ring fitting portion to be fitted in a chamber-side fitting portion of the vacuum chamber and a second ring fitting portion to be fitted in a pump-side fitting portion of the vacuum pump; a foreign particle entrance prevention member provided at the ring main body; and a dropping prevention structure configured to prevent the center ring from dropping from the pump-side fitting portion of the vacuum pump.