The bulge test apparatus designed for the measurement of mechanical material properties of thin films was constructed and tested. The principle of the test is to apply pressure on a free-standing membrane, to measure the membrane shape and to analyse the results. Commercially available silicon nitride (Si3N4) thin films were used for the testing. It is shown that interferometric set-up designed and assembled for the apparatus enables precise determination of 3D shape of the whole membrane, which allows more precise