PURPOSE: To rapidly and accurately detect the increase of a pressure in an oil tank case and to prevent the occurrence of a leakage trouble of exhaust gas and a breakage accident of a pump.

CONSTITUTION: An opening end is located under an oil level in the oil tank case 3a of an oil rotary vacuum pump 1 and a pressure gauge 20 is attached to a pipe member 19 led out in a leak tight state from the oil tank case to the outside of a pump. Based on an output signal from the pressure gauge, a relay contact for protecting and stopping a pump and an abnormal signal are taken out. A piping for oil bubbler is sued as the pipe member. Further, it is preferable that a reference pressure based on which pressure abnormality is detected by the pressure gauge 20 is set to a range of approxmmately 1.5-3kg/cm.