First and second annular valve faces that approach closely or contact against first and second annular valve seats on valve cases are formed on both the upper and the lower ends of an annular valve body portion of a main valve body, an air intake chamber is defined by a piston reception hole and a piston portion, and the auxiliary changeover valve changes over the main changeover valve between its air supply position and its air discharge position by supplying compressed air to the air intake chamber or discharging air therefrom. A center side portion of the main valve body is made as a separate auxiliary valve body. The sealing of a second valve member is released by the auxiliary valve body being rapidly shifted with low pressure compressed air, and thereby the main valve body is reliably changed over.