A drug delivery device comprising a delivery unit (2) including an internal reservoir (8), a subcutaneous delivery mechanism (6), a pump for pumping liquid from the internal reservoir to the subcutaneous delivery system, and a liquid filling mechanism (12) configured for injecting liquid from an external vial (16) or reservoir into the internal reservoir. The liquid filling mechanism comprises a filling port (14) and a valve mechanism (18) selectively operable to: open a first fluid path (54) extending from the filling port to the internal fluid reservoir and close a second fluid path (56) extending from the internal reservoir to the subcutaneous delivery member, or close the first fluid path and open the second fluid path. The valve mechanism comprises a valve housing (48) fixedly mounted in relation to a housing of the drug delivery device and a valve stem (50) rotatably received within the valve housing, whereby the first fluid path and the second fluid path are selectively operated by the rotational position of the valve stem in relation to the valve housing, such that when the valve stem is in a first position, the first fluid path is open while the second fluid path is closed, and when the valve stem is in a second position, the second fluid path is open while the first fluid path is closed.