The present invention relates to a Roots pump. It utilizes a pair of rotating shafts to make several rotors to extract gas. Every rotating shaft can be extended and passed through a rear shell memberof said Roots pump. It includes several ring shaft seals, every ring shaft seal is mounted on the periphery of its respective rotating shaf and is received in the notch formed in rear shell member interior, and a spiral groove is formed on a peripheral side surface of every rotating shaft, and when the related rotating shaft is rotated, every spiral groove can be used for driving the oil between the peripheral side surface of related shaft seal and peripheral wall of the notch and making it move from one side correspondent to the pump cavity towards the gear receiving cavity, so as to effectively prevent the oil from leaking into pump cavity.