Disclosed is a dosing unit (1,2,3) for an ambulatory infusion device, comprising a stationary member (20,120) and a movable member(30,130,330) which make, in combination, a charging valve and a discharging valve. The dosing unit further comprises a plunger (50,150,350), located, at least in part, in a metering cavity (31,131,331) of the movable member (30,130,330) and being adapted for displacement in the metering cavity (31,131,331), the plunger (50,150,350) being in a threaded engagement with the movable member (50,150,350). The plunger (50,150,350) is adapted to couple continuously to a driving unit (505) and the movable member (30,130,330) is adapted to selectively couple to the driving unit (505), such that• by operating the driving unit (505) with the movable member (30,130,330) being coupled with the driving unit (505), the movable member (30,130,330) may be moved with the plunger position along the displacement axis (A) being maintained and that• by operating the driving unit (505) with the movable member (50,150,350) being decoupled from the driving unit (505), the plunger (50,150,350) may be displaced with the position of the movable member (50,150,350) being maintained.
