The sanitary mix proof valve can be operated by compressed air. The normally-closed valve is equipped with two miniature normally-open pneumatic valves, namely, a detecting valve and a valve for CIP. The valve plug contains two seals which connect with each other to form a leakage chamber. Any leak composition may flow into the chamber and outflow through the detecting valve. When the pneumatic actuator is supplied with compressed air, CIP will be conducted. During cleaning, flow direction is over against the plug, which makes the valve not susceptible to water hammer.