Abstract: A seal device for a gate valve is disclosed, in which annular members facing the front and rear surfaces of a gate valve member having a through hole are coupled to a valve cage by inner and outer bellows, each of the annular members has an annular recess formed on the side facing the gate valve member and closed by an annular mirror surface finished thin plate secured by welding to an opening thereof, the individual bellows are elongated by fluid under pressure supplied into annular spaces defined by the inner and outer bellows, and the annular mirror surface finished thin plates are strongly urged against the gate valve member by the pressure of the fluid forced from the annular spaces into the annular recesses. Edge portions of the annular mirror surface finished thin plates are to be very firmly mounted to the annular members, and the mechanical strength of the thin plates is to be increased.