Flanges (10a) of an attachment plate (10) integrally combined with an ion detector (5) are fixed to an inner surface of a vacuum chamber (1) via cushioning members (12), such as O-rings. Although the vacuum chamber (1) mechanically vibrates due to a turbomolecular pump attached to it, this vibration is absorbed by the cushioning members (12), whereby the vibration of the ion detector (5) is suppressed. As a result, the noise which occurs with a vibration of the ion detector (5) and is superposed on the detection signal is reduced. Thus, the quality of the detection signal can be improved by a simple and inexpensive structure.