Metal gasket and vacuum flange

A vacuum seal structure comprising a ring-shaped metal gasket (11) with a substantially rectangular cross-section, with the ratio (t/w) of the thickness (t) of the ring portion to the width (w) thereof being from 0.5 to 4, and having a Vickers' hardness of from 50 to 120, and a vacuum flange (23) for a metal gasket, including a sealing groove for receiving a metal gasket at the surface of the flange, the width of a cross-section of the groove gradually decreasing in the direction of the depth thereof, an outer side wall section of the sealing groove being substantially perpendicular to the flange surface, and an inner side wall section of the sealing groove being inclined with respect to the flange surface, with a difference in level being provided so that the flange surface positioned to the inside of the sealing groove is lower than the flange surface positioned to the outside of the sealing groove.