A back flow preventing device is disclosed which prevents the reverse flow of water from a point of use into a water supply system. The device has a casing with coaxially aligned inlet and outlet openings, and containing coaxially aligned inlet and outlet check valves. The coaxial alignment of the inlet and outlet openings, and of the inlet and outlet check valves serves to minimize the pressure drop through the device. Between the inlet and outlet check valves is a diaphragm actuated relief valve which opens to drain the passage between the check valves if they should malfunction or if there is a drop in the supply line pressure sufficient to cause actual or potential reverse flow back toward the supply line. The internally-located inlet and outlet check valves are each removably secured in place by internally-located retaining rings.