To provide an over-fastening preventing means in a simple and sure form in a device for hydraulically operating a vehicular cover flap, etc., which is provided with a double acting working cylinder, in which the working cylinder is pivotally mounted to a vehicle on one side and to the cover flap on the other side, in which a working chamber of the working cylinder is connected to each one side of selective pressure sources via a check valve, in which both check valves are provided with devices for cutting off/releasing the partner each other, and in which each working chamber is placed parallel to the check valve and is kept at a prescribed level via a preload valve whose discharge into a tank is controllable.

A conduit 22 is connected to a tank from at least one of connecting conduits 4, 5 leading to a working cylinder, a proportional pressure valve 23 is inserted into the conduit 22, so that the proportional pressure valve 23 is controlled by an electronic controller in correspondence to the force just necessary for the motion of a cover flap or the like.