A vacuum measuring cell has a first housing body and a membrane, both of Al.sub.2O.sub.3 ceramic or sapphire. The membrane is planar with a peripheral edge joined by a first seal to the first housing body to form a reference vacuum chamber. A second housing body of Al.sub.2O.sub.3 ceramic or sapphire opposite the membrane, is joined to the peripheral edge of the membrane by a second seal to form a measurement vacuum chamber. A port connects the vacuum measuring cell to a medium to be measured. At least in the central area of the first housing body, an optical transparent window is formed and at least the central region of the membrane has an optically reflective surface. Outside the reference vacuum chamber, in opposition to and at a distance from the window, an optical fibre is arranged for feeding in and out light onto the surface of the membrane.