A device for the manual metering of a medical fluid, such as a hardening resin of the bone cement type, comprises a chamber for the fluid to be metered (2, 102) with an end opening (6), a gripping handle (7, 107), a piston (10, 110) which is housed in the chamber (2, 102) and provided with an operating member (12) for a user, and screw-type actuating means (13) arranged in between the operating member (12) and the piston (10) so as to perform the longitudinal movement thereof and pressurise the fluid to be metered. The screw means (13) may be modified so as to change from an operative configuration, where the piston is fed micrometrically, to an inoperative configuration, where the piston may move freely. The screw means (13) comprise an outer thread (14, 114) associated with the piston (10) and a female-thread element (15, 115) formed integrally on the handle (7, 107); the handle may be deformed elastically by means of transverse compression so as to change the screw means (13) from their operative configuration to their inoperative configuration.