A seal structure has one metal gasket arranged between a pair of casings. The metal gasket is provided with a planate outer peripheral portion and a bead portion which is formed by rising up from an inner peripheral end portion of the outer peripheral portion, and is arranged so that the bead portion comes into contact with the one casing, and the outer peripheral portion comes into contact with the other casing. The bead portion elastically deforms so as to reduce its height and the outer peripheral portion elastically deforms on the basis of the elastic deformation of the bead portion, when the metal gasket is pinched by the pair of casings, and a leading end of the outer peripheral portion comes into contact with one casing at this time, whereby any gap in which salt water stays is not formed between one casing and the metal gasket.