Abstract: A control line connects an annular space in an auxiliary hydraulic cylinder of an hydraulically-movable mine-roof support (the auxiliary cylinder being provided to advance a forward, auxiliary, roof-support canopy in the direction of the working face of a mine) to the displacement space of an hydraulic measuring cylinder which is longitudinally displaceable in a direction parallel to the hydraulic advancing cylinder in order to advance the auxiliary roof-support canopy, which is pivoted to the main roof-support canopy, in synchronism with the advancing cylinder mounted in the base of the mine-roof support and attached to an abutment on the working face side of the support. A control valve is connected to the control line and responds to the control line pressure in such a way that, when the pressure falls, it supplies pressurized fluid into the pressure space of the auxiliary cylinder to extend the piston rod thereof, and shuts off again when the pressure increases.