PURPOSE: To prevent an impeller from inclination and its boss portion from intrusion of foreign matters by providing upper and lower ends of the boss portion of impeller with annular lips and making seal portions on inner periphery of both ends of the boss portion bear against the outer surface of a drive shaft and further ribs contact with resiliently the drive shaft.

CONSTITUTION: Since annular lips 38, 39 are provided on upper and lower ends of a boss portion 32 of an impellet 30, foreign matters are blocked by these lips 38, 39 to reduce an amount of their intrusion into the interior of the boss portion 32. Also since seal portions 43, 44 formed on the inner peripheral surfaces of both ends 30a, 30b of boss portion 32 bear against the outer surface of a drive shaft 8, the interior of the boss portion 32 is prevented from the intrusion of foreign matters. Further, since ribs 45... are provided to resiliently contact with the drive shaft 8, the impellet 30 is not inclined with respect to the drive saft 8, and slide of inside surface of pump chamber on the lips 38, 39 and resilient contact between the seal portions 43, 44 and drive shaft 8 are ensured so that the interior of the both portion 32 can be prevented from the intrusion of foreign matters.