To provide a rotor drive mechanism decreasing a dimension of a pump device in a longitudinal direction, also decreasing volume of fluid storage space within a casing, and improving a discharge rate.

In the rotor drive mechanism 25 for transmitting rotation of a drive shaft 38 which is rotated around a constant position to an external thread-type rotor 22 of a uniaxial eccentric screw pump 23 via a connecting shaft 39, the drive shaft 38 has an inner space 46 which opens toward the rotor 22 side, the connecting shaft 39 is inserted to the inner space 46, an aft end of the connecting shaft 39 is connected with the drive shaft 38 via a second joint 48, a forward end of the connecting shaft 39 is connected with the rotor 22 via a first joint 47, and the space between an inner peripheral surface of an opening of the drive shaft 38, oriented toward the rotor 22 side, and an outer peripheral surface of a rotor shaft 37 coupled with the rotor 22 which eccentrically rotates, is sealed by a first seal portion 55.

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