PURPOSE: To prevent an alternator housing from the intrusion of lubricating oil by providing a negative pressure introducing groove on an inner wall surface facing the pump side of a side plate of a vacuum pump.

CONSTITUTION: In a side plate 4 provided on the rear end surface of an alternator housing 2, a negative pressure introducing groove 14 is formed on the inner wall surface facing the pump housing 11 side, the inner end of said groove corresponds to the inner end of a guide groove 13 of a rotor 12 and the outer end communicates to the negative pressure section of pump. In the rotation of rotor 12, most of lubricating oil discharged to a vane 14 moving inward in the guide groove 13 is discharged to the outer negative pressure section of pump through a negative pressure introducing path 41, so that the lubricating oil hardly intrudes in an oil seal 5. On the other hand, since the oil seal 5 section communicates to said groove 41 through a fine gap between the side plate 4 and the rotor 12, oil pressure in this section is maintained under the negative pressure. Thus, the pump side of oil seal 5 maintains the negative pressure and the fluctuation of pressure is restrained small to display satisfactory sealing property.