A positioning device is e.g. used for positioning the door of a motor vehicle in an opened position. The positioning device comprises e.g. a cylinder and a piston rod which extends inward and outwards of the cylinder through one end thereof. A flank face is provided at the inner wall surface of the cylinder. The piston rod carries a piston. The piston is provided with a counter-arresting ring which engages the flank face in a predetermined position. When the piston passes across the arresting flank of the cylinder, the counter-arresting ring is moved radially inwards with respect to the axis of the piston rod against elastic resistence. So, an increased resistance occurs against the axial movement of the piston rod with respect to the cylinder. This increased resistance against axial movement positions the door with respect to the body frame of a motor vehicle in a predetermined opened position and avoids that the door is closed unintentionally. Af the flank face and the arresting ring are enclosed within the cavity of the cylinder, they are not subject to environmental conditions, which might deteriorate their positioning function after a period of use. Lubricating agent may be provided within the cavity for lubricating the flank face and the arresting ring. As such lubricating agent is enclosed within the cavity of the cylinder, there is no risk for the vehicle driver to be polluted by the lubricating agent.