Abstract: This disclosure relates to a metering dispenser which dispenses fluid in accurate proportion to the pump revolution speed, and to an improved shaft seal for the motor drive shaft of a metering dispenser. An elastic sleeve membrane is mounted inside an expanded area provided in a part of the peripheral wall of the pump casing so that a constant-pressure chamber is formed to the outside of this sleeve membrane, and the pressure inside the pump casing is maintained within a set pressure range by introducing a pressurized fluid into this constant-pressure chamber and releasing the pressurizing fluid from the constan-pressure chamber as needed.