Abstract: The balloon inflating device comprises a syringe (12) the plunger (28) of which has a screw thread (36). It further comprises a mechanism for retaining the plunger comprising a first half nut (40A) movable between a position separated from the screw thread (36) and a position of engagement with the screw thread (36), and a slidable control element (42) which comprises first coupling means (54A) adapted to displace the first movable half nut (40A). The retaining mechanism (14) further comprises a second movable half nut (40B) movable between a position spaced away from the screw thread (36) and a position of engagement with the screw thread (36). The control element (42) further comprises second coupling means (54B) associated with the second half nut (40B) and adapted to move the second half nut. The first and second coupling means (54A, 54B) are mechanically connected to each other in the region in which the control element (42) is coupled to the first and second half nuts (40A, 40B).