In order to prevent gaps from occurring in corner sections in a section where three surfaces are joined together, comprising the combination of a housing, grommet and cover, or the like, thereby improve sealing, a seal structure, which is provided with a housing in which a notch-shaped grommet-mounting section is disposed, a grommet, a cover and a metal substrate gasket, is provided, wherein the grommet is provided with a seal projection, and the seal projection forms a protruding section due to a portion of the seal projection protruding toward the metal substrate gasket when the seal projection makes intimate contact with the inner surface of the grommet-mounting section. The metal substrate gasket is provided with two seal lines and a groove-shaped recess between the seal lines. When nipped, the protruding section of the seal projection is tightly compressed inside the groove-shaped recess in the metal substrate gasket.