With this device, it is possible to open the car door (30) of a lift car located in the area of a floor together with a shaft door, and in fact in the normal case automatically driven by a drive (36) or, in the event of a power failure, manually by a passenger. If the lift car is located outside a floor, the car door remains locked by the same device. The car door (30) is driven by a band-shaped drive means (42) via a driving parallelogram (1) consisting of a rigid and a movable cam (2, 3) and arranged on the top part of a car door. For the free travel, the driving parallelogram (1) is closed; it opens for coupling to the shaft door between two coupling rollers, arranged on the shaft door, before movement of the doors takes place.; The movable cam (3) of the driving parallelogram (1) has a compressible stop cam (5) which is lifted from a rigid cam carrier (4) by leaf springs (6) during the travel and during holding outside a floor. During the coupling with the coupling rollers (44, 45) of a shaft door, the stop cam (5) is pressed against the rigid cam carrier (4). This displacement is utilised for unlatching a car door latch (12).