PURPOSE: To detect a state of deteriorated oil, by separating a gaseous component dissolved in the oil of an oil rotation pump through a permeation membrane, and determining it with a mass spectrometer.

CONSTITUTION: The entitled analyzer consists of an oil rotation pump 1, an oil circulation pump 2, a filter 3, a device 4 of a permeation membrane 15 made of silicone rubber, a mass sepetrometer 5, and a valve 6 for joining the device 4 with the spectrometer 5. The device 4 consists of a vessel 11 provided with tubes 12, 13 connected with the oil circulation system of the pump 1, and furnished with a deflector 14 for deflecting oil flow toward the membrane 15 in the vessel. The membrane 15 is supported with a support 16 having its central part 16a made of porous metal or metallic oxide.