A volumetric flowmeter (1) is equipped with a detachable pump unit (2), a main body casing (3) accommodating the pump unit (2), and a cover member (4). The main body casing (3) is equipped with a front side main body casing (8) accommodating the pump unit (2) and integrated with a differential pressure detector (6), and a rear side main body casing (10) connected to the front side main body casing (8) and allowing mounting therewithin of a servomotor (9) constituting a main body of a shaft driving unit (5). The pump unit (2) is inserted into a unit accommodating recess (11) of the front side main body casing (8) and is then covered with the cover member (4) to be thereby completely accommodated.