An apparatus for setting a gap between glass substrates (13, 14) of a glass panel (P), wherein the glass panel is clamped by a constant force regardless of its thickness. The apparatus includes a flexible and annular movable plate (4) biased by coil springs (5, 5) toward an annular immovable plate (1) so as to clamp the glass panel (P). The immovable and movable plates (1, 4) have a first gasket (3) and a second gasket (6) respectively to clamp the glass panel (P). At an outward part of the first and second gasket a third gasket (8) is disposed between the immovable and movable plates at a level such that the distance between the bottom of the second gasket (6) and the top of the first gasket (3) before the glass panel is placed between the first and second gaskets (3, 6) is smaller than the thickness of the glass panel when the biased movable plate (4) presses the third gasket (8). To obtain a desired gap between the glass substrates (13, 14), the air between them is evacuated by a suction pipe (10) through a chamber (S) defined by the glass substrates, first, second, and third gaskets, and immovable and movable plates. The suction pipe (10), at one end, where the chamber is located, has a flow control valve (11) to obtain a desired rate of air flow by suction.