PURPOSE: To simplify the structure and assembly by disposing an inner part of the control rod in its entirety within the cross-section contour of an outer part, and making the maximum diameter of the inner part including the coupling members thereof smaller than the inside diameter of the outer part.

CONSTITUTION: A control rod 1 is supported within a pump casing 2 and coupled with an adjuster 3. The control rod 1 is divided into inner and outer parts 4, 5. Coupling means formed by grooves 7, 8 extending laterally with respect to an axis 6 are formed in inner and outer parts 4, 5 respectively. In this case, the inner part 4 is disposed in its entirety within the cross-section contour of the outer part 5. The maximum diameter of the inner part 4 including the coupling members 7 thereof is set to be smaller than the inside diameter of the outer part 5. Thus, the structure of the control rod 1 is simplified and its assembly becomes easy.