An external seal structure in a vacuum valve in which an upper end plate of a partition wall member is held between a valve body and a cylinder cover. An annular seal member for main fluid is placed between the upper end plate and the valve body to prevent material gas from leaking to atmosphere through an abutting section defined by the valve body and the cylinder cover connected in abutting manner. The main-fluid annular seal member is made of perfluoroelastomer. A fluorinated annular seal member made of fluorinated resin is placed between the upper end plate and the valve body and between the main-fluid annular seal member and the abutting section. A leak port communicated with outside air is provided in the valve body to communicate with between a seal point of the main-fluid annular seal member and a seal point of the fluorinated annular seal member.